20 Most Powerful X-Men Villains Of All Time
3. The Dark Phoenix
A running theme in the X-Men books is that not only are mutants their own worst enemies, but the team's greatest threats specifically come from within their ranks. Onslaught was pretty bad, but that was still Xavier working with his own power set. The Phoenix Force, meanwhile, took Jean Grey's existing psychic abilities and added a whole lot more.
It's still not fully explained but the Phoenix Force is a cosmic energy which searches for a host and endows them with the powers of a god. Something which, apparently, doesn't do wonders for your mental state. Because as soon as Jean Grey got the powers of a god, she immediately utilised the Phoenix's ability for destruction.
The Dark Phoenix Saga's most horrifying moment is when Jean-as-Phoenix wipes out an entire civilisation in a single bound, taking millions of lives in the blink of an eye. She was only stopped when Jean wrestled control of her mind and begged Cyclops to kill her – otherwise, she would never have been stopped. That's true power, and truly scary power.