Hmmm, well, maybe we're going about this all wrong: instead of trying to mod him into HFS, it might be better to mod him into a special event on the surface: place the Temple Of Armok in atop the highest mountain on the map. But just knowing the location isn't going to immediately land you a dual with Armok: the temple would be guarded by a dragon, and inside the temple is a tunnel, going down several Z-levels, into the very heart of the mountain, guarded by megabeasts, bloodmen (who we might mod a little and rename Slaves To Armok, and give them iron armor and weapons), and some other nasty surprises. Once you reach the bottom (or is it?), you will have found the Avatar of Armok himself!
After some thought, the idea had occurred to me that making Armok a creature of flesh and blood (even divine flesh and blood) wouldn't work perfectly. Instead, we can take a page out of the Bloodmen's playbook: we can make him completely out of modded blood. This blood neither freezes nor boils, and it has the exact temperature of boiling (human) blood. But the kicker? He's wearing adamantine armor but no weapons: that's right, you'll be fighting Armok in a wrestling match. With no thought center, eyes, bones, nerves, or any of the weaknesses of other creatures, even if you do hit him, you're not going to kill him easily. And even if you win, there's a tunnel going further down, to nearly the HFS itself, and at the bottom, you will come to another large chamber, with Armok Himself, a creature similar to the one you fought above, but gigantic. However, you don't HAVE to fight this one (unless you want to). Talk to him, and he will bestow upon you one of the following:
The Wrath Of Armok: A legendary hammer, unbelievably heavy, more powerful than any other in the world. A swing from it is like a hit from Armok himself. Send foes flying with a single swing!
The Aegis Of Armok: Legendary Armor, no force on earth, not even arrows, can pierce it.
(Both of these are heavy in the extreme, naturally.)
The Offerings Of Armok: A fortune, more than you can spend in a thousand lifetimes.
The Aid Of Armok: Armok joins your party. From now on, Armok will aid you in vanquishing your foes.