The Phoenix Force possessed Jean Grey and others, most of whom ended up calling themselves Phoenix. It ultimately feasted on stars orbited by populated worlds, a crime against sentient life for which Grey paid with hers. Then things got really complicated because comics, but let's not go there. When the Phoenix Force returned to Earth, the X-Men allowed it to possess five of them instead of just one, on the theory that this would weaken it enough for them to remain in control of its destructive tendencies. This plan, as you might expect, worked out great for everybody with no negative consequences whatsoever.