Edit: Oh wow I meant to say before Skyrim but said before Morrowind... Think it's all my hard work with a previous mod... Wearing me out and making me a little absent minded, edited that little error out.

I'm on a quick little project right now, trying to make Daedra, and all of the ethereal/ghostly creatures of Skyrim totally immune to steel, iron, wooden, and imperial weapons, and ideally unarmed attacks too. It lets me alter nearly any other race, giving it a spell which triggers a perk that grants the supernatural normal weapon immunity ability. Because frankly, that's one of the major reasons that TES games before Skyrim had creatures that felt more... Supernatural, and magical, cause of that normal weapon immunity, and it made getting a weapon of silver or better quality, or sometimes just being a spell caster, feel like more of an accomplishment.

I made Dragon Priests, Flame Atronachs, Frost Atronachs, Storm Atronachs, Witchlights/Swarms, Wisps, and Ice Wraiths all immune to normal weapons, as it just seemed... Very appropriate to me. The last race that appears in vanilla Skyrim I wanted to give this ability to, was the Dremora race. However, whenever I give Dremora this ability, it shows in the ability slot, but when I click okay, the Dremora race shows as unaltered, and then the spellist on it's race general tab is no longer there. I was then going to make a version for the ghostly/daedric creatures of Dragonborn and Dawnguard as well, if it had any.

Can someone please tell me why this is happening, and what the best away around this is?