This happened to me when I was experimenting with astral projection and other occult activity. I believe the dream was more than a mere simple dream.
As always, the pitter-patter of rain and the booming of thunder had lulled me off to sleep. I dreamt of something that night that I really can't explain in words, but I'll try my best. I was floating in the space of space. The stars sparkled around me and almost seemed to dance like snow falling on a winter's day.
I got curious as to why I was floating in nothingness and examined my hands. They were not the fleshy things they are now, but were giving off an ethereal glow. I had somehow managed to astral project in my sleep! I let out an excited "yes!" because I had been trying this for awhile without success.
As I moved about the emptiness, I noticed a planet that was very similar to Earth. The clouds wisped around it like streamers, and its bluish green tones were beautiful beyond belief. My stargazing was cut short when I saw a swirling vortex coming toward the planet. There was nothing I could do but watch the beautiful orb get devoured by the swirling darkness.
Then the darkness spoke in my mind in almost a monotone: "This is the fate of all worlds. I am one of many devourers. Your planet and its inhabitants will suffer the same fate, just like the inhabitants of this planet."
Images flashed into my mind of people that once existed on the world that was just destroyed. They were different from us in appearance, almost reptilian, but had similar structure to their cities and ways they moved about in their daily lives. Tears streamed from my eyes as I watched mothers holding their children tight, people running away among large buildings in a fruitless attempt to escape the swirling darkness that they had seen coming.
"The universe is just like any other ecosystem," it said. "I am coming for your planet. Everything that your civilization has accomplished will be meaningless."
I woke with my clothes dampened by cold sweat. I stepped out of bed to get a glass of water but instead placed my bare feet out onto the patio and stared up at the stars. Its voice came again, "I'm coming for your planet." Then I realized that the nearest black hole is within the Milky Way galaxy, and that it's in the constellation of Sagittarius.