You do not get Desolate Dive from a Snail, rather you get it from Soul Master, who likely experimented on one - considering how the Shaman mentions an ill-tempered third uncle.The Shaman never seems to become suspicious and believes they are all voluntary gifts (which may or may not be true in two cases).
However, nothing ever confirms this or if the snails had anything to do with the Pale King's efforts to keep Radiance away and forgotten, barring the Soul Sanctum experimenting on one in their own efforts. Unfortunately, only one of them remains during the course of the game. A tribe of snail creatures knowledgeable about the Soul and capable of channeling spells using it.
