(The gentleman below was originally written up however many years ago for my homebrew/Wilderlands setting, some of which I’m recycling for the AD&D Underworld game.)
The abominable Glaath-Yigon is one of the most powerful wizards of the City of Faz, ruling a small personal Underworld beneath the forsaken Tower of Uldarus. The High Dingus suffers the Necromancer and his unsavory activities in return for certain unique services and provisions. In his dark little realm, he performs ghastly experiments and releases many of the products thereof to roam the tunnels and vaults. Other wizards speculate that the Necromancer is not only proficient with magic but with certain forms of ancient technology the operation of which is lost to all others.
Glaath-Yigon is horrifying and inhuman, his blasphemous studies having irrevocably warped his physiognomy. He is swollen and bloated, his soft body livid with bruises and settled fluid. His visage is repellent: his head is outsized and gourd-like, crowned with tangled black hair; a ragged hole gapes where his nose once grew; his ears are large and jutting, bat-like; his toothless grin is a slick purple grotesque. He casts no shadow.
Squatting on his hovering pedestal, foul Glaath-Yigon wears no clothing save a simple breech-cloth covering his unspeakable nether regions, which he plies upon a steady succession of unfortunate slaves. Despite his apparent indolence, the Necromancer is a fearsome warrior on those rare occasions when he is stirred from his perch, leaping and waddling with unnatural quickness. In his arrogance, he will often forego his powerful sorceries for the thrill of locking his filthy fingers around a throat.
Glaath-Yigon has developed many new forms of undead, including the deviant hormads, the skeleton-giant hekons, and the abhorrent zols and vrlus.