Right.
Now to figure out how to make a creature that can't be killed. Slade flesh or something . . .
I made an "Archon" race that, as far as I've seen, is uninjurable (Except maybe by another Archon, and during world-gen where they have the durability of wet tissue paper.) The secret lies in making new organic materials so that every piece of their body is made of something far stronger than even adamantine. They can also swim in lava and don't breathe, so that's no danger either. In one test, I sent about 50 adamantine bow and crossbow men and various other adamantine armed races after a single Archon in the Arena.
They were all slaughtered. The Archon was never even scratched. And that was before I had them pick up a weapon. About the biggest inconvenience they ever suffer is getting knocked down by something that's a much larger size. Megabeasts are also quickly slaughtered by them.
Still a WIP though as I can't figure out how to get certain appendages to function as proper slashing weapons (They are tagged as "EDGE") so they aren't as deadly as they'd be if they could decapitate things without weapons.
Maybe curses or something could do something to them, I don't know how those work or how you would make something immune to them.
I can post what I've got if you want.