Quantum didn't notice it moving on its own.
It is VERY hard to kill the Unfettered. When it's moving on its own, you can't ignore it. However, it still has few very big flaws:
1) It only does trample damage. It has very low AP for a trampler. Further, it can get Limp. It just won't kill things. They'll rout, of course, but that doesn't give it any corpses.
It will have MAJOR difficulties fighting ethereal, high-defense and/or regenerating creatures. Because it WON'T kill things fast, it might hit the turn 50 battle limit, and rout, and die.
2) It doesn't kill population. Pazuzu, Lord of the Plague Wind, has disease wind. Eater of the Dead doesn't. Making Eater kill population would fit it much better than a high-level mage that won't be spending ALL its time in enemy lands.
3) It starts battles with just 352 hp. I think that's due to even the Unfettered's base HP being 180. It does regenerate up to 1000+ hp pretty quickly.
It does have few nice abilities, though. Wonder how it "gets up to 1000+ abilities pretty quickly"? Hmm...