This is a theory that seems to talk about Bowser and King Boo from the "Mario" series.
Bowser is a crazy villain. Some of his deaths at the end of Mario games are pretty brutal. An example is in Super Mario Galaxy 1, when he falls into a star and has to go through an exploding planet that turns into a black hole with him on it.
King Boo's role in the Mario series at first glance doesn't seem to be anything significant. He only appears in few Mario games and, aside from the Luigi's Mansion series, his role in the games he does appear in is minor.
You may think that Bowser and King Boo have nothing related to each other. However, they both play a major role together in the Mario series which keeps the series alive. While this is never explained directly, there are a few games in the Mario series which give hints about the truth of Bowser and King Boo.
Super Mario World
Super Mario World is the first game in the Mario series in which hints about the theory are included. While it is small, it is important, especially for King Boo.
King Boo was really around as early as the Super Mario Bros. 3 days in which Boos first appeared. King Boo was around in this time, too, he just never appeared in-game. While Super Mario Bros. 3 doesn't include hints to the theory, Super Mario World does.
Have you realized how occasionally there were these "Ghost House" levels in Super Mario World? These houses are where Boos lived peacefully, and they didn't want anybody bothering them. Then came Mario, who randomly ran into the house, disrupting and sometimes knocking out Boos. This happened numerous times in most Ghost House levels. The Boos knew they had to stop this, so they reported to King Boo and told him that Mario has been "a pain" to them and he needs to be stopped. King Boo was furious about this, as Ghost Houses are meant to be places where Boos could live peacefully, but instead Mario keeps starting chaos within the Boo families that reside there. Thus, King Boo started tracking down Mario.
This started the Boo's and King Boo's hatred toward Mario that continues to today.
Paper Mario is the second game in the Mario series that provides hints about the theory. This game is very important to the theory, especially for Bowser. While the main attention in this game goes to Bowser, King Boo deserves to be noticed as well. Between Super Mario World and Paper Mario, Mario continued to be disruptive to Boos, especially in Yoshi Games. While he hasn't been as disruptive, it was still enough to continue King Boo's hatred toward Mario.
In Paper Mario, though, Mario actually allied with the Boos in World 3. While you may think this would change King Boo's mood about Mario, if you think more into it, you'll see that what Mario did wasn't nearly enough to make up for what he did in Super Mario World. First, Mario only allied with one Ghost House, compared to the several he disrupted in Super Mario World. Second, Mario was kinda forced to help the Boos, because they wouldn't give him the Star Spirit until he helped solve their problem. All in all, what Mario did in Paper Mario wasn't enough to stop King Boo's hatred toward him.
Bowser played a much bigger and interesting role to the theory in Paper Mario. The end of the game is where it gets interesting.
This starts in the scene where Bowser and Kammy Koopa are on a platform about to explode. Mario is on it, too, but he escapes due to the help of the stars. Bowser, though... the explosion of the platform is heard, explosions are seen, and Bowser is not seen anywhere escaping. Note that the platform is also thousands of feet in the sky, if not more. While Bowser's deaths before this game were a little understandable on how he survived, (Maybe he can swim in lava and only got injured on his falls.) there is no explanation on how Bowser can survive being on a exploding platform thousands of feet in the sky. The explosion or, at least, the fall should have killed him.In fact, proof about Bowser's death may be hinted in-game. At the very end of the game when you must walk to Peach's Castle to trigger the ending scene, go to the center part in town.
In front of Merlin's house should be a sign, which is called the: Toad Town News. After mentioning about how Mario defeated Bowser and how Peach and her castle are safe, it also mentions this:
Bowser was badly beaten and will likely never return.
We're sure of it.
The sign clearly states how Bowser was beaten badly, which is true. Being on an exploding platform thousands of feet in the sky is literally a death sentence, even for someone as tough as Bowser (Mario also fell from the same height at the beginning of the game and got killed, but he was shortly revived by the stars) The sign also states on how Bowser won't return. This is also true. There is no way Bowser can return if he is beaten up so badly that he is likely dead.
But wait, if Bowser is "dead," then why is he still alive as the main villain in the Mario series today? This is because the theory still expands to the Mario game after Paper Mario, Luigi's Mansion.
Luigi's Mansion is perhaps the most important game to this theory. Both King Boo and Bowser play equally important roles in this game. This game is why the Mario Series still has Bowser as the main villain today. While this is never told, it is hinted. The hints are actually shown right in front of you. It's just that it's hard for one to connect all of them together. Fortunately, I was able to.
You all know what happens at the start of the game. Luigi goes to a mansion that he "won in a contest" only to find out that it is much darker than what he was shown in a provided picture. He also doesn't know where Mario is, who went into the mansion before him. Luigi meets Professor E. Gadd, who gives him the Poltergust 3000. This is the first game where King Boo is seen in-game. He first appears after Luigi opens a trapdoor and a bunch of Boos and King Boo fly out of it.
About mid-way through the game, you see King Boo in a fancy room with Mario inside a painting, struggling to get out. This is the first sign that King Boo is up to something evil.
But, soon after, one of the biggest hints about this theory is told. In Area 3, do you remember the "Fortune Telling Ghost?" Also known as "Madame Clairvoya." You know, the kind ghost who tells Luigi to bring Mario's dropped objects so she can see what happened to Mario? Well, she says something very important.
Once you give her the last object, after she says to the orb to show them all it can about Mario, she will say this:
Wh-what is this!?
How can this be?! I see the hideous form of Bowser! Is Bowser somewhere in this mansion? I cannot believe it...
And yet I see it... I thought that Mario had soundly defeated Bowser!
Has King Boo somehow revived Bowser? This could be horrible! Well, for you.
The Fortune Telling Ghost mentions important things. She confirms that Bowser still exists, but she doesn't understand why considering Mario soundly defeated Bowser. Remember that Paper Mario was the Mario game before Luigi's Mansion and how the Toad Town News sign in Paper Mario also thought that Bowser wouldn't return. Also remember Bowser's brutal defeat at the end of Paper Mario. See a connection here?
Also note the part when she mentions how King Boo could have revived Bowser. This will be important later on.
The next hint and connection occurs when Luigi opens the door to King Boo's room. Before the battle begins, King Boo does some talking about Mario's painting. He does, however, mention a connection:
I remember how much trouble you've (Mario) caused me in the past.
This doesn't make sense at first, considering this is King Boo's first appearance in the Mario series, but remember what I said earlier about how Mario kept on going into Ghost Houses in Super Mario World? That and other rude interactions with Boos is what King Boo meant by the "trouble."
After some talking, King Boo somehow makes the room do a 360 and make the Mario painting turn into a Bowser painting. King Boo then goes into the Bowser painting and sucks Luigi in. Then you will fight the final boss on the roof of the mansion; Bowser with King Boo inside of it.
Remember, connections. Remember how the Fortune Telling Ghost mentioned on how King Boo could've somehow revived Bowser? All in all, King Boo wanted to get revenge on Mario for the trouble he did with other Boos by luring him into a mansion filled with ghosts and reviving his now dead arch-enemy; Bowser.
After Luigi's Mansion
The connections don't stop after Luigi's Mansion. If you pay attention, you will see that Bowser's deaths after Luigi's Mansions are more violent and extreme then before; especially in the more recent games.
For example, as mentioned before, in Super Mario Galaxy 1 (Released in 2007.) Bowser fell into a star. This should surely burn him to death, even if he can swim in lava the gravity should flatten him. Yet he appears later on a planet that is getting destroyed. The planet then explodes and turns into a black hole with Bowser on it.
It may seem like there is no way Bowser can survive that and other brutal deaths in recent Mario games. However, there is one way to explain this.
Just recently I explained on how King Boo revived Bowser. Well, he didn't get completely destroyed in Luigi's Mansion. Why? He is a ghost, and as you all know, because ghosts are already dead they can't die. What King Boo did instead, though, is give Bowser some help. He took over his body to keep him alive, hoping that he can find a way to get revenge on Mario.
You know how ghosts come for revenge and forever haunt you. This is what is happening to Mario. King Boo is taking over Bowser to keep him alive. Since ghosts can't die, King Boo is thinking of even more evil plans then before.
This explains why Bowser's plans recently are normally more extreme than just capturing Peach. For example, in most games before Luigi's Mansion, Bowser's plan is mostly to just capture Peach and keep Mario from saving her. In games after Luigi's Mansion, Bowser focuses on trying to really marry Peach (Perhaps King Boo has an infatuation with Peach), actually killing Mario, and to conquer the Mushroom Kingdom. Sometimes he even takes it to another level to conquer the Universe (Like what was seen in Super Mario Galaxy). Bowser's sudden darker personality in games after Luigi's Mansion proves that he is controlled by King Boo, as King Boo has proven to have a dark, evil personality in his appearances in-game, similar to Bowser's dark personality in Mario games after Luigi's Mansion.
This is also why Bowser keeps coming back, no matter how brutal his defeat is. Since he is controlled by King Boo he won't die as he is now part ghost. You may be thinking it is Kamek or Bowser Jr. who is reviving Bowser instead. That could be true in some of Bowser's less extreme deaths (Such as falling in lava).
However, in post-Luigi's Mansion games, some of Bowser's defeats at the end of the game are so extreme that it is hard to believe Kamek can actually revive him. For example, in both Super Mario Galaxy games, Bowser's final defeat is getting sucked into a Black Hole, and it is hard to imagine Kamek or Bowser Jr. is going to go into the Black Hole with him to revive him (They will get killed as well). Only King Boo, a ghost who can't die, can help his body somehow find a way to survive still intact. Perhaps he even made another copy of Bowser's body if his body was lost in the Black Hole. Not even Kamek or Bowser Jr. can do that!
Bowser's body could have got beaten extremely badly and lost at the end of Paper Mario, thus Kamek could have not revived him. Only King Boo could create a new copy of Bowser's body and control him himself if this is the case, as he could have done as the final boss in Luigi's Mansion.
In fact, in some of the more recent Mario Bros. games, such as New Super Mario Bros. for the Nintendo DS, a new form of Bowser called Dry Bowser appears, which is basically Bowser with only his bones. He appears after Bowser's body was previously destroyed in battle, mostly from falling into lava. It seemingly makes no sense for Bowser to have his skin burnt off by the lava and still function perfectly with only his bones. The reason for this is that Dry Bowser is actually Bowser's carcass (The remains of Bowser after he is defeated) in which King Boo is possessing. Thus, Bowser still seems perfectly functioned with only his bones because King Boo is possessing his bones and his body remains. Bowser's carcass is actually not the only body in which is possessed after it is defeated. Dry Bones enemies, which are the remains of regular Koopas, are able to still function and be a threat to Mario despite being dead because regular Boos are possessing them. Since King Boo is more powerful than the other Boos, he is able to return Bowser's skin and flesh to his body to make him more complete. He is also able to defend Bowser/Dry Bowser against a normal Dry Bone's worst weakness (Fire) in some way with his increased power when compared to other Boos.
After Luigi's Mansion, King Boo only appeared in Mario spin-off games such as Mario Party and Mario Kart. While he does also make appearances in Super Mario Sunshine and Super Princess Peach, this could be because King Boo left Bowser's body to destroy Mario by himself.
The more likely reason for Bowser and King Boo appearing in a few Mario Games together though is the fact that the King Boo in non-Luigi's Mansion games is just a fake while the real King Boo is controlling Bowser's body. Notice how the Luigi's Mansion King Boo has special features such as the glowing eyes, purple tongue, and the weird crown. The King Boo in other Mario games doesn't have any of these features. So most likely, the King Boo you see in Mario Kart, Mario Party, and other non-Luigi's Mansion games is just a fake and replacement while the true King Boo is busy controlling Bowser. Dry Bowser does appear as a character in Mario Kart Wii also with the regular complete Bowser, but it is possible that the Dry Bowser in that game is either a fake or a clone made by King Boo.
Luigi's Mansion 2: Dark Moon
In Luigi's Mansion 2: Dark Moon, King Boo escapes from Bowser's body as he realizes that Luigi is again capturing ghosts. (Bowser doesn't make an appearance in that game, which also is a connection.)
He decides to attempt to destroy Luigi again; this time in his real ghost form without Bowser's body. Despite his best efforts, Luigi again spoiled King Boo's plans to take over the Mushroom Kingdom in his ghost form.
King Boo of course doesn't really die. He just returns to Bowser's body to think of his next plan to get revenge on the Mario Bros.
Before Luigi's Mansion, Bowser is really Bowser himself. He wants to capture Peach and take over the Mushroom Kingdom. However, he fails, and instead gets hurt from falling into pits and swimming in lava, and perhaps even revived by magic in some of those cases. In Paper Mario, though, after his plan failed, Bowser once and for all is destroyed as he deals with an explosion on a platform thousands of feet in the sky. The explosion was so extreme that Bowser's body was forever destroyed and lost and so not even magic could revive his body. It appeared like the Mushroom Kingdom and Peach will be safe once and for all.
However, King Boo had different plans. King Boo had a hatred toward Mario for all the trouble he had caused with his Boos in games such as Super Mario World. He wanted revenge on Mario. He even perhaps has an infatuation with Peach, and wants to find a way to marry her. King Boo found the chance when he found out about Bowser's death. He decided to capture Mario for himself and revive and create a new Bowser body to take over the Mushroom Kingdom for himself. Thus, he could get his revenge on Mario and marry Peach, Bowser's body hiding the fact that he really is a ghost.
Luigi, though, saved the day as he destroyed King Boo and his revived Bowser and saved Mario. However, as you all know, ghosts don't die. King Boo took over Bowser's body and controlled him. He continued with Bowser's plans. Bowser's plans after Luigi's Mansion are more evil than before because of this. Also, because Bowser is now controlled by King Boo, a ghost that is already dead anyway, Bowser keeps on coming back for more, no matter how brutal his death is. King Boo a few times does leave Bowser's body to destroy the Mario Bros in his real form, but the Mario Bros. are too much for him, as his plans keep getting spoiled by them.
All in all, King Boo is currently really controlling Bowser's body with many future goals in mind waiting to be accomplished.
This is continuing to this day. King Boo is thinking of his next plan to marry Peach, get rid of the Mario Bros. and take over the Mushroom Kingdom once and for all.
Super Mario 3D World
In Super Mario 3D World, instead of capturing Peach, King Boo instead attempts to kidnap the 7 Sprixie Princesses. This makes no sense, but it actually does. The reason is that the Sprixies are the only ones who can actually destroy ghosts. This leads to King Boo kidnapping the Princesses instead of Peach. Although defeated, King Boo was still able to escape. After being defeated by one or more of our heroes, King Boo used the defeat as a diversion to return to their world and, convincingly, trapped in a bottle.
Another similar theory to the one explained above is that King Boo is actually Bowser's ghost. The theory would remain largely the same, except this time, there was no true King Boo before Paper Mario, and when Bowser died in Paper Mario, his spirit became King Boo. Therefore, the trouble in the past that King Boo meant in Luigi's Mansion would not just be for the trouble he caused in Ghost Houses, but the trouble he caused him while he was alive trying to capture Peach.
King Boo then built and revived his own body, which, with the exception of Luigi's Mansion: Dark Moon, used to try to get revenge on the Mario Bros. over and over but with failure each time. With the fact that Dry Bones are referred to as dead Koopa Troopas possessed by their own spirit in the form of Boos, it makes sense that Bowser, the king of the Koopa Troopas, would be possessed by his own spirit in the form of King Boo, and Bowser also having a Dry Bones-like form with Dry Bowser.
This could also better explain how Bowser, even after Luigi's Mansion, showed an infatuation toward Peach. Since under this theory King Boo is the dead form of Bowser, it would make sense that he would still try to marry Peach, but take it more seriously now that he is undead.