Amulets of life saving only generate when a lich generates, either in its inventory or somewhere else on the level where it first generated. Each amulet is tied to its lich, and the lich can only be permanently killed when its amulet is destroyed (including by the player using it up to save their own life, but not limited to that; any method of destroying it such as looting it from a cursed bag of holding would work). If the amulet remains intact, the lich will eventually resurrect.

If a lich is genocided, any amulets connected to liches of that type aren’t destroyed, but stop being effective to save lives. Possibly there should be some visible indicator of this, such as the amulet being cracked. If all liches are genocided, then no amulets of life saving will work.

Not specified what would happen if an amulet of life saving is wished for; probably either the amulet doesn’t work, or it spawns a random lich it’s connected to somewhere on the level.

When a non-unique warper (and possibly some unique warpers, like non-magical quest nemeses) is cancelled, they cease to be able to warp. This means that levels in Gehennom can contain caged liches that are cancelled and thus incapable of warping out, or of spellcasting for that matter.

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.