EDIT:after reading the comments, this is what i think will be a be better ending than the lolkill i proposed:
somewhere in the middle of the story, the protagonist and antagonist face off, the antagonist not being at his strongest, because he has not preformed the ritual needed to release the power of the artifact that he stole. the antagonist is naturally overpowered, and sees that should the protagonist not be slowed down, he will not have the time to do so, so he puts a curse on him, that in time will kill the protagonist. he also during this battle tell, that were he to be stopped a much greater evil would befall the world, but no one believes him naturally, after that, he teleports away.
despite the curse, the protagonist presses on. during the last battle he wins just by a thread, and as he does, he succumbs to the curses effects. as the rest of the party emerges from the battle, the greater evil starts invading.
*credit roll*
is this better?