Perhaps in this fanfiction, Achilles discovers a prophecy that if Troy falls, it will lead to the destruction of his own homeland in a different way. So, he fights for Troy not just as a hired warrior but as someone with a vested interest in the city's survival. He uses his incredible combat skills to fend off the Greek invaders, and along the way, he forms unexpected alliances with some of the Trojan soldiers who were initially skeptical of him.
In the 'Achilles Fights for Troy Fanfiction', it could be that Achilles is resurrected after his death in the original story by a powerful Trojan sorcerer. The sorcerer convinces him that his true destiny was to fight for Troy all along. With a new - found purpose, Achilles trains a group of young Trojan warriors, sharing his combat knowledge. He then leads them into battle against the Greeks, using his rage in a more controlled way to gain an edge in combat.