Maybe something where an NPC recruits the party to help them cleanse the Sacred/Cosmic beasts. These creatures represent the Zodiac and also various domains (think clerics). You can have a variety of encounters. Some are battle, some are puzzles, some might require special rituals and defending a point from infinite waves, etc. In the end, you find the Zodiac Beasts were actually sealing a cosmic horror (Cthulhu or what have you) who is actually the NPC. With the beasts defeated his power returns and he turns on the party. Bonus if the NPC changes after each beast is defeated in subtle ways, becoming stronger and more mysterious/*wrong*. The risk of course is the party figuring out what's happening early. This is fine. Have the NPC run off and hire new mercenaries to take over. The party will work to stop them. Assuming they will stop some, adjust the number of beasts needed for the seal. For example, if they never catch on then they defeat all 12 (13?) if they do catch on, then six failures might be enough for the NPC to forcibly break it's bindings. Or 8. Or whatever number preserves the tension will still allowing the party to successfully stop the mercs a few times.
Ooh I really like this path! The cleansing is a great idea, they could go either that route or killing them and have different kinds of rewards for both outcomes. I love the idea of it breaking a seal for the npc, that makes more sense for the slow descent into madness than gathering pieces for a ritual at the end like I was thinking. I absolutely want a fakeout bad guy to draw their attention away from whats happening, maybe someone they end up being able to partner with at the end.