Maybe sort of like berserk where it gives more attack and defense, but maybe like it takes your health constantly
Which you can restore by either killing stuff and having the armor eat them, or just health potions I guess
And also ike instead of an armour slot, it's like a side slot or something, which gives you a skill you can use to switch in and out of the form.
And to make sure the player isn't too OP early on t'd be like one of those things where you have to finish the Big Mama and Elise questline to the end.
Then maybe need you to like go into Sybaris to require some special abomination or something, which also needs the help of Lisa and her explosives so you need to finish her questline too.