Dissimortuum

Spider Climb. The dissimortuum can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Undead Nature. The dissimortuum doesn't require air, food, drink, or sleep.

Multiattack. The dissimortuum makes three Claw attacks. It can replace one Claw attack with a use of Terrifying Mask.

Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 15 (3d8 + 2) slashing damage.

Terrifying Mask. Each non-Undead creature within 60 feet of the dissimortuum that can see it must succeed on a DC 15 Wisdom saving throw or be frightened for 1 minute. A frightened creature can repeat the saving throw, ending the effect on itself on a success. If a target's saving throw is successful or the effect ends for it, the target becomes immune to the dissimortuum's Terrifying Mask for the next 24 hours.