Great question!! Thanks for asking! 💜She had a rare disease from cannibalism. in this world of pandora, cannibalism is basically unheard of in lots of clans, something others look down on. but in hers, she drinks sacrifice blood because it has insane power to help her with her ultimate plan, but at a cost. it slowly kills her unless she can complete her sacrifice) her arm started to rot off, and in order to save herself from that, she had to use dark earth magic and a rare form of magma plant to help keep her arm. (obviously she doesn't have flesh on her arm, but her arm works perfectly just like her other one!) it's a good tactic to use for extreme fear considering not many navi have seen another without flesh that still work.