They don't attack the dead. The dead don't have what they want.

[But also, as annoying and powerful as they are, these monsters seem... lackluster.]

Still, I don't think it's them he wants. They're not much of a prize... So that means there's something else down there, something valuable enough for him to besiege this place for almost a year.

... The water itself, maybe? It looks... familiar, doesn't it?