Revisitation
The Mutant X Havok did not die during the fight with the Sentinels. He was instead thrown into unconsciousness while the Earth-616 Havok took over his body. The evil Havok went with the original Havok back to Earth-616 during the final battle with the Goblin Queen. He lay dormant when Charles Xavier freed Alex's consciousness, biding his time, and re-emerged after Xavier Institute Student Nicholas Gleason (Wolf Cub) wounded Havok. Luckily, the Exiles (Mimic, Magik, Heather Hudson, Nocturne, Morph and Sunfire) were sent by the Timebroker to fight off the evil Alex until the Earth-616 version came back. The Timebroker then personally eliminated the evil Alex's consciousness.
Read more about this topic: Mutant X (comics)