At least part of the final conflict of the story features a contest between the main protagonist and the prime antagonist minion, essentially the bad guy’s chief henchman.
The minion starts the story as a friend of the main protagonist, if not one of the protagonists themselves. Somewhere along the line, the minion abandons and perhaps betrays the protagonists to become the minion; this may be the event that leads to the dire consequences for the protagonists at the end of act two.
Assuming that the minion acts with free will, his conversion from adversary of the antagonist to minion must occur over time for a reason. The antagonist must be espousing a philosophy or course of action that, ultimately, the minion finds compelling enough to allow him to abandon his former friends, perhaps even betray them. At the very least, the antagonist must be offering the minion something that he cannot turn down.
Before he becomes the minion, this character should be, in many ways, the most powerful of the protagonists, the one the others look up to in some way. He should have abilities that make him seriously attractive to the antagonist, and the antagonist should court him through the introduction and the complication, ultimately winning him over to his cause.
The minion, once he turns, becomes a highly ambivalent character. On the one hand, he is dedicated to his new cause — he must be, because of the high price he paid to embrace it. On the other hand, he still feels friendship with the protagonists. Being in conflict with them causes him considerable emotional distress. When the final conflict comes, though, he is completely committed to it, willing and able to destroy his former friends.