well if they are cubes they could be souls of celastians like maybe malistaire came wanting to learn the astral schools and raised 3 souls then turned them into cubesafter he was done with them.he probably forgot to return them to the grave when we came along at the tutorial.
Them too because didn't you read their books! Myth draws power from fire and ice, death draws power from storm and ice, life draws power from fire and storm while balance draws power from all of them so if you think about it all magic decends from the dragons,giants and tritons!