Dark Phoenix was the amoral manifestation of Phoenix in the X-Men. Pushed to the psychological brink by the mental machinations of Mastermind, Phoenix's moral sense broke down and she severed her ties with her own humanity. She defeated all the X-Men with ridiculous ease, flew around the Sun, and opened a stargate to a distant galaxy, where she promptly caused a star to supernova in order to absorb the resultant energy. When a Shi'ar vessel attempted to challenged her, she destroyed it. When she returned to earth, Professor X contained Dark Phoenix within mental barriers and she resumed her Jean Grey persona.
It is interesting to note that the various power fluctuations associated with Phoenix were not an issue for Dark Phoenix. This has led some to suppose that the limitations on Phoenix's awesome power were caused by the moral sense and personal doubts of the Jean Grey persona, and that when these were removed by Mastermind, Phoenix felt no constraints.