Upon ascending the rank of the Spirit Runer and leaving the title of the Rune User, they will gain numerous upgrades and benefits. One of them is turning Body Modification into Body Transformation!
Body Modification takes a toll on energy but Renir didn't notice this back then because he was always replenishing that energy and always using Body Modification.
Now that he was more aware, he didn't use it in the early game. But when the Archangels began to move, Renir has no choice but to rely on it.
He just hoped that he will get lucky and consume at least an Angel.
But he would think of that later on. The important matter right now is what his Body Modification had become.
Unmeig's Body Transformation changed him into a seemingly whole different being while Aavron's was a bit chaotic and uncomplimentary due to the forceful way of ranking up. Using it too much would make him self-destruct.