The EM has primary responsibility for this narration. But in practice, either the EM or the treasure hunter can describe things. Just as when you're playing other RPGs, sometimes the GM describes what happens when a PC attacks a goblin, sometimes the player does. Probably different groups will find different ways that they think are comfortable: some favoring the EM as storyteller, some the other players, some treating this as purely a guessing game and not an RPG at all, so having very few fictionalized details. I think that all should be fine, so long as everyone's on the same page.