The Revenant Sycophant is a Revenant Horror / Zombie Slayer5Mini-Boss that has a chance to be spawned when a player with an active Revenant HorrorTier III Quest kills a Zombie-type Mob. When this happens, an explosion effect takes place and the Revenant Sycophant spawns.
The chances of spawning one can be increased by the player's Combat XP multiplier. The XP multiplier can be increased by the player's Slayer combat multiplier, Viking's Tear potions, and/or a legendary Wolf Pet. Jerry event potions count toward slayer combat XP.(Source?)
It runs somewhat quickly but its damage output is not very high if the player is wearing decent armor (around 500❈ Defense should be enough to tank several hits).
When killed, they give 360 Combat XP, which is as much XP as around 11 Crypt Ghouls, meaning they will make the player much closer into summoning the boss.