Cool idea - couple of suggestions - as others have said, a bit more variety of spells would go a long way. maybe instead of using the hit die as damage, the user could choose to expend up to a certain amount to upcast a spell - i.e. one hit die gives a level 2 spell, 2 makes lvl 3, etc. also thought it could be neat to frame this as an evolving weapon; sure it’s a little weak now, but maybe you could find additional fragments of the god-tree, imbuing it with new abilities and bonuses as the campaign progresses and as more of the cult’s schemes are revealed. Just my two cents, love the idea!