Betrayal

NPCs, even allies, can shift allegiances. They might betray, fail, or sell out their companions, which can make for a meaningful story event. If an NPC is being set up to betray the party in some way, lay groundwork early on so the players don't feel ambushed by the twist. If the players can look back and see a clear path to this result, it's likely they'll feel the decision makes sense in the context of the story. Try to give the NPC a “tell” or a paper trail that can be found, especially one where the players have chances to pick up on that tell with smart use of their abilities and skills.