Amnesia Demon

Magic Resistance.  The Amnesia demon has advantage on saving throws against spells and other magical effects.

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

Never seen. If a creature makes a Wisdom perception check to find an Amnesia demon they know is in an area and fails the check, they must make a DC 17 Wisdom saving throw or forget the Amnesia demon exists.

Multiattack. The Amnesia demon can make two bite attacks and a forgotten thought

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 16 (3d6 + 6) Piercing damage. 

Forgotten thought. The Amnesia demon forces a creature within 30ft to make a DC 17 Intelligence saving throw. On a failed save the creature takes 7 (4d6) psychic damage and forgets how to use its last used class feature, weapon proficiency, or spell. If multiple of these were used in the same action the DM randomly rolls for one of the traits. On a successful save, the creature takes half as much damage and doesn't lose the traits. Traits lost in this way are regained on finishing a long rest, this feature cannot remove proficiency in unarmed attacks or race abilities. 

Repelling Curse: If a cursed enemy moves closer to the demon, they have to make a DC 15 dex saving throw or be pushed back 10 ft. Once per target per turn.