Cancelled, non-unique warpers (and possibly some unique warpers, like non-magical quest nemeses) shuts down their warping. This means that cages in Gehennom can contain liches that are cancelled and thus incapable of warping out.

Add a unique lich or demon lord enemy who may appear as a side effect of raising the dead with the Book of the Dead. Not specified if this is the only way this monster can ever appear, or whether other mechanics like demon gating could bring them in as well.

When a nasty is summoned by a lich, and its monster type has a zombie counterpart, it will instead be replaced by that zombie counterpart.

Remove/replace the guaranteed L and V monsters from Astral; the priests shouldn’t tolerate random high level undead hanging around.

New special room “lich hall”. Contains one difficulty-appropriate or somewhat-out-of-difficulty lich, generated asleep with difficulty-appropriate undead. One or two statues generate along the walls, and there are several spellbooks on the floor or in chests. Within the hall, the lich can summon more undead.

Whenever a lich is created, a special phylactery amulet is created somewhere on the level. When the lich is killed, the phylactery begins a timer to resurrect into the lich again. The player can somehow destroy it to finish the lich off once and for all.

Liches drop or are generated with fairly good magical items, like potions, scrolls, spellbooks and even wands, to incentivize not genociding them.