this is a good idea, i would think that if you were to do this, you'd want to combine it with the skill uncapper and instead make devil trigger a lesser power that chews your magicka away and turns off when the magicka is gone, there'd need to be an upper limit to the timescale in DT mode however to keep balance. the skill uncapper comes into play when skills and attributes need to go past 100 through demonic buffs perhaps? the graphical side of the mod could be accomplished through using polymorph scripts such as werewolf and wolf in midas magic. a different form for male and female of the race perhaps. and also have like water breathing / shield / hp + fatigue regen and magicka resistance 40% whilst in devil mode? perhaps a permanent stunting of magicka regen like an atronach sign would be appropriate and provide the neccessity for alchemy and high use of potions