As a founding member of the X-Men, Jean Grey has steadily proven to be one of Marvel's single strongest characters. Considered an Omega-Level mutant with no discernable power limit, Jean is capable of bringing down mystical threats like Nightmare and can stand her ground against Celestials, even without possessing the awesome might of the Phoenix Force. It's a good thing for the Marvel Universe that she's (usually) a noble and heroic figure, otherwise, the whole world could be in danger.
This isn't the case for all versions of the character, however. One notable variant of Jean Grey became a multiversal marauder, using her awesome might to take over entire worlds with relative ease. Here's how one Jean Grey variant became a multiverse-level threat when she remained an evil figure - and how she did so without ever becoming the Dark Phoenix.
An Evil Jean Grey Took Over an Entire Reality
Across the multiverse, variants of Jean Grey have proven their might against some of Marvel's most powerful beings. A completely unrestrained version of the character proved capable of holding her own against the cosmically powered World Devourer, Galactus. Even the ruthless reality-warper Korvac considered her enough of a threat that she needed to be removed from his reality for him to accomplish his goals. Luckily for the multiverse, most versions of Jean Grey are heroic, although the exceptions to the rule highlight just how dangerous she could actually become. This includes a version of Jean who committed untold crimes in her home reality and proved vicious enough to warrant being exiled from her own timeline.
Left adrift across dimensions, this Jean eventually made her way to Earth-998, a reality ruled by the British Empire at the turn of the 20th century. This monarchy had been under the control of Dark X-Men leader Madelyne Pryor for years, leading to a period of prosperity for the empire. When she eventually passed, her body was burnt as part of a prophecy regarding her eventual return. Seeing an opening to seize power from a society that deeply wanted to believe a myth, the villainous Jean used her physical resemblance to Pryor to steal her name and title, becoming a vicious leader with a lethal attitude.
The Evil Jean Grey Was Immensely Powerful
Despite ruling an entire world, this villainous version of Jean Grey had greater ambitions. This included luring variants of Nate Grey (otherwise known as X-Man) to her world and killing them to claim their power during the "Shock Wave" storyline that ran through X-Man #67-70 (by Steven Grant, Warren Ellis, Ariel Olivetti, Christie Scheele, and RS & Comicraft's Troy Peteri). One of the targeted Nates included the primary version of the character, who was able to work alongside one of his variants to unite against their common enemy. Notably, the newly dubbed "Queen Jean" showcased how deadly she could really be. With a simple thought, the queen was able to cause people to explode from the inside - something she did with casual cruelty against the minions that failed her. She could easily read the minds of other powerful telepaths while carrying on full conversations. She could even pull the life force from others to empower herself, slaughtering her own soldiers during her battle with Nate Grey.
The prospect of a truly evil Jean Grey is terrifying, especially when she has the might of the Phoenix Force at her disposal. The fact remains that any version of Jean is already an Omega-Level mutant, powerful enough to decimate entire worlds and become a multiverse-level threat. Even more frightening is the fact that the experience and power levels of the core-Marvel Universe Jean Grey make her easily capable of bringing forth the same level of destruction as her out of control counterpart, and it is only her heroic qualities that keep her devastating powers at bay.