When the Fantastic Four and the Avengers sacrificed their lives to destroy Onslaught, Franklin Richards, the son of the FF's Mr. Fantastic and Invisible Woman subconsciously created a pocket universe to save their lives. There they spent an entire year reliving living new lives "reborn" into a reality where they recently gained their abilities that vaguely mirror how they gained their powers originally.
On Counter-Earth, Kang claimed to be a time traveler from the 30th Century, however since the pocket dimension of Counter-Earth was artificially created by Franklin Richards, and the fact that the Gamma core on Avengers Island proved to be a rift in space-time that was spawning villains for the Avengers to fight, Kang's claims may be a fabrication. Whatever the truth may be, he and his lover Mantis would travel to the modern era on Counter-Earth in order to battle the greatest heroes of all time, the recently formed Avengers. Kang wished to utterly defeat them as a token of love to Mantis.
