Phoenix Force

The Phoenix Force is a powerful cosmic energy. It was tasked with protecting the M'Kraan Crystal. It found Jean Grey and possessed her after she apparently sacrificed her lift to protect the X-Men. It put her in a coma forcing Jean into a hospital. Phoenix then got the X-Men to help her help Lilandra of the Shi'ar stop her brother and take control of the Empire. It then sent the M'Kraan Crystal into a star to protect it. However, it took Jean with it causing grief to Cyclops and Wolverine. Phoenix then returned to Earth but wouldn't leave Jean. Her emotions overwhelmed Phoenix and it wanted to experience more. Jean became jealous when Cyclops spent time with Dazzler. It then joined the Inner Circle Club as the Black Queen but fought against the group. It defeated the X-Men and went out into space calling attention to Uatu, Doctor Strange, and Thor. It destroyed a star system causing the Shi'ar to confer with the Skrull and Kree about killing Jean and the Phoenix. The X-Men got Phoenix under control but they were confronted by the Imperial Guard. Charles Xaveir issued a challenge for Jean's life so his team fought the Guard. Most of the X-Men were defeated but Phoenix stopped them all. However, Jean was apparently killed. Phoenix left her body and told all there that together they could restore her life. The X-Men and Imperial Guard agreed and Jean was alive again.

Background
Phoenix, like Jean Grey, was voiced by Catherine Disher.

In the comics, Phoenix actually did kill a planet full of people. It then destroyed the Shi'ar ship rather than disable it. In the final battle, Jean Grey sacrifices her self to stop the Phoenix. Phoenix was originally meant to be an extension of Jean's powers. It was later retconned to become a separate entity so that Jean could come back to life without guilt. The series was made with this change in mind.

In the comics, Phoenix has also possessed Charles Xavier. Rachel Summers, seen in the alternate Age of Apocalyse universe, does possess an alternate Phoenix but this is not directly stated in the series. In alternate universes, it has also possessed Cyclops, Nightcrawler, and Colossus (who was transformed into a woman).