I'm thinking mutiny. The remaining crew realize they're in this boat (pun intended) because of the leader. Half of them are dead. The remainder don't ever know if they'll ever get home again, but it sure doesn't look like it. The main folks who would step up and defend the leader are nicely tucked away in a tomb. While the leader sleeps, the crew kill him. Put his head on a spear and basically desecrate the rest of his body sort of thing.
No need to do the time thing. Allow normal time to pass. When the party emerges they'll find the leader's head, but not the rest of the body. They'll also have on their happens a mutinous crew who looks at them as partially to blame. And unless the party is able to win over the crew immediately, the mutiny is going to threaten to consume them, too. Of course, even if they do win over the crew, there will always be the seed... meaning you have a simmering plot line.