Achtryn is a demon who temporarily replaces the Slayer Master Mazchna while he is busy during the quest While Guthix Sleeps. Like Mazchna, Achtryn is a demon, though his face and proportions look decidedly human. He claims to be from the abyssal regions, where, it is said, demons on Gielinor originate from. After the quest is over, Mazchna returns to his post near Canifis, and also reappears on the NPC Contact spell in the Lunar Spellbook, and Achtryn is not seen again.
He claims that his lair in the abyssal regions is a fine place of residence, but it is good to see the sights, to meet mortals and generally socialise.
As his temporary replacement, Achtryn maintains the same assignments, weightings, and challenge as Mazchna.