As they're walking, Legion says aloud "Hmm... you know, it doesn't really make much sense to go to that much trouble to bury that dagger in such an elaborate way... If someone thought so much of that dagger, why wouldn't they just keep it on them, and use it?"
He's silent for a moment, then continues "What if the cell wasn't for the dagger, but the dagger was part of a prison for something else? Something terrible... After all, the dagger is what was generating the ice."
He stops walking, and looks to Sorrel and says "I think you should send your birds to look around in there more, to make sure there's not something else that we missed buried in that ice..."