Despite the name, a demilich was not a lesser lich, but rather a lich who had evolved beyond a need for its undead body.

The target must make a DC 19 Charisma saving throw. A demilich has a way to regain its soul as shown in the sidebar disiples of acererak. Acererak is best-known as the architect of the Tomb of Horrors, which has earned a reputation as being one of the most dangerous dungeons in the history of Dungeons & Dragons. Magic Weapons: The sphinx's weapon attacks are magical. Those Demiliches whom are Disiples of Acererak are CR 21 outside their lair and CR 23 inside their Lair and have Gems for Eyes. Combat. As a demilich, he moves to the demiplane of Moil to complete his grand plans. Dungeon Magazine issue 213 had a conversion of Tomb of Horrors to D&D Next (the 5e playtest rules) done by Chris Perkins. Acererak's Prepared Spells. On a failed save, the target's soul is magically trapped inside one of the demilich's gems. Wisdom (Insight) checks made to ascertain the sphinx's intentions or sincerity have disadvantage. I was reading the Demilich entry in the MM, as its regional effects are probably the closest to something like this.

Notably, 5e changed a lot of the lore surrounding demiliches, so maybe the writers decided that it no longer made sense for Acererak to be a demilich. Acererak's aura attack is double the strength of a normal lich's and is saved against at -4. There are 32 creatures with truesight and fly speeds across all officially published material, according to D&D Beyond's monster listing.Their monster listing has options to filter by a number of things, including special senses and types of movement speeds.
Occasionally he will appear as a full lich, but always with a ruby red skull and bejewelled eyes and teeth, a scimitar covered in constantly oozing blood on his side. Acererak or another demilich like him has a challenge rating of 21 (33,000 XP), or 23 (50,000 XP) in its lair, and gains the following additional action option.

The target is allowed a fortitude saving throw (DC 36). by Mike Shea on 20 November 2017. In 4e and 5e, liches become demiliches when they forget to feed souls to their phylactery.
I've got a copy here and I'm going room by room to update the traps (in many cases increasing damage since Pathfinder characters have significantly greater HP) and make sure … And yet, we are constantly seeing DMs lament that the final fight with Acererak in Tomb of Annihilation was disappointing. Apprentice to Vecna who attempted to subvert the will of the plane of negative energy to subsequently control all undead planeswide, failed and became a vestige. Specifically, you can permanently enslave Acererak, the demilich, in his Tomb of Horrors. The Tomb of Horrors is long Acererak's home in undeath. There is a shrine of Acererak, in the form of a five-foot statue of a humanoid skull, on the second layer of Pandemonium.

The real Tomb of Horrors, not the heavily wussified version released for 3rd edition. Such a campaign deserves an equally deadly final boss. Trap Soul. 32 creatures with truesight and fly speeds. The final chamber of the Tomb involves a battle with Acererak in his demilich form, where he appears as a floating skull, with gemstones for eyes. This doesn't really make sense for Acererak as this definitely isn't something he'd forget to do.