Avoidance.

If the demilich is subjected to an effect that allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Legendary Resistance (3/Day).

If the demilich fails a saving throw, it can choose to succeed instead.

Turn Immunity.

The demilich is immune to effects that turn undead.

Actions

Howl (Recharge 5–6).

The demilich emits a bloodcurdling howl. Each creature within 30 feet of the demilich that can hear the howl must succeed on a DC 15 Constitution saving throw or drop to 0 hit points. On a successful save, the creature is frightened until the end of its next turn.

Life Drain.

The demilich targets up to three creatures that it can see within 10 feet of it. Each target must succeed on a DC 19 Constitution saving throw or take 21 (6d6) necrotic damage, and the demilich regains hit points equal to the total damage dealt to all targets.

Legendary Actions

The demilich can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The demilich regains spent legendary actions at the start of its turn.

Flight. The demilich flies up to half its flying speed.
Cloud of Dust. The demilich magically swirls its dusty remains. Each creature within 10 feet of the demilich, including around a corner, must succeed on a DC 15 Constitution saving throw or be blinded until the end of the demilich's next turn. A creature that succeeds on the saving throw is immune to this effect until the end of the demilich's next turn.
Energy Drain (Costs 2 Actions). Each creature with in 30 feet of the demilich must make a DC 15 Constitution saving throw. On a failed save, the creature's hit point maximum is magically reduced by 10 (3d6). If a creature's hit point maximum is reduced to 0 by this effect, the creature dies. A creature's hit point maximum can be restored with the greater restoration spell or similar magic.
Vile Curse (Costs 3 Actions). The demilich targets one creature it can see within 30 feet of it. The target must succeed on a DC 15 Wisdom saving throw or be magically cursed. Until the curse ends, the target has disadvantage on attack rolls and saving throws. The target can repeat the saving throw at the end of each of its turns, ending the curse on a success.

Lair Actions

On initiative count 20 (losing initiative ties), the demilich rolls a d20. On a result of 11 or higher, the demilich takes a lair action to cause one of the following effects. It can't use the same effect two rounds in a row.

The tomb trembles violently for a moment. Each creature on the floor of the tomb must succeed on a DC 19 Dexterity saving throw or be knocked prone.
The demilich targets one creature it can see within 60 feet of it. An antimagic field fills the space of the target, moving with it until initiative count 20 on the next round.
The demilich targets any number of creatures it can see within 30 feet of it. No target can regain hit points until initiative count 20 on the next round.

Regional Effects

A demilich's tomb might have any or all of the following effects in place:

The first time a non-evil creature enters the tomb's area, the creature takes 16 (3d10) necrotic damage.
Monsters in the tomb have advantage on saving throws against being charmed or frightened, and against features that turn undead.
The tomb is warded against the magical travel of creatures the demilich hasn't authorized. Such creatures can't teleport into or out of the tomb's area or use planar travel to enter or leave it. Effects that allow teleportation or planar travel work within the tomb as long as they aren't used to leave or enter the tomb's area.

If the demilich is destroyed, these effects fade over the course of 10 days.

Demilich