IIRC, Auric Ulvin is the only unit in the game with the race/class (I forget which it is) Avatar. Though Meshabber, The AoW, and Mithril Golem are all called "avatars", I believe they are all "melee" class units with the race demon, demon, golem, respectively. I might be slightly off on that, but the gist is as follows: Kael and the FfH team kinda made an error by denoting AA as an "avatar". He's a Deity, and the _God_slayer kills deities. Avatar might be a more exact term for him in the English language, but it is inconsistent with the rest of the pedia.
So there's really no reason for the Godslayer to do anything but kill Gods. Well, Magister can correct me if I misinterpret who signed the Compact.
Either way, I would find it kinda odd if the Godslayer/Netherblade killed immortals. I mean, the most common immortal unit in the game is the "Immortal", who I thought was always undying in the sense of the Persian Immortals, i.e., faceless, nameless, highly trained soldiers whose numbers were always kept exactly the same after every battle (via the induction of new recruits), creating an illusion of immortality for friend and foe alike. If that's how the generic Erebusian immortal unit works, the relative nature of the weapon used to kill them shouldn't matter.