Deathly Choir. Anyone within 10 feet of Rahadin who isn't shielded by Mind Blank hears, inside their head, the screams of the many he has killed. As a Bonus Action he can make each creature hearing them attempt a DC 16 Wisdom saving throw; on a failure it takes 16 (3d10) Psychic damage. The engine does this at the start of his turn, to his enemies only.
Fey Ancestry. Rahadin has Advantage on saving throws against the Charmed condition, and magic can't put him to sleep.
Mask of the Wild. Rahadin can attempt to hide even when he is only lightly obscured by foliage, heavy rain, falling snow, mist or similar natural cover.
Actions
Multiattack. Rahadin makes three Scimitar attacks, or throws two Poisonous Darts. The engine runs the three swings; at range he throws one dart.
Scimitar. A quick, precise cut.
Poisonous Dart. Throws a poisoned dart up to 60 ft.
Innate Spellcasting. Rahadin casts spells using Wisdom (spell save DC 15), needing no components. 3/day each: Misty Step, Phantasmal Killer.