The Phoenix, or Phoenix Force, from the X-Men comics of Marvel, is a cosmic entity that most famously possessed the merry mutants' own Jean Grey and went a little off the rails. If by 'a little off the rails' one means eats suns and wipes out an entire planet.
But despite this pretty epic of dining faux pas, the Phoenix is more than just a one-dimensional destructive force of nature. And yet, it seems that that is all anyone remembers of the being.
You destroy one race of sentient broccoli people...
So what else is the Phoenix about? What more is there to know about the cosmic firebird, that fans and perhaps even writers, might have forgotten? With Phoenix returning to the comics and part of next year's X-Men film, it's never been a better time to remind ourselves.
The general manifestation of the Phoenix Force is that of a large firebird, as the name might suggest. But the Phoenix is more than just flames and fire and pyrotechnics.
The Phoenix Force is actually the manifestation of life itself, and the potential for life in the universe. As such, it's capable of all sorts of things, from resurrection to even correcting the timeline and the course of life itself.
Galactus himself once commented upon the nature of the Phoenix as an entity of life essence, and suggested that the more the host uses the Phoenix Force, the more they were taking from the potential for life itself in the universe.
So yes, flame and fire to be sure, but also a fountain of the very life essence that pervades the universe and populates planets in every galaxy.