Devotion of His'tuun is an aberration created with the sole intent to spread the "good" word of His'tuun - an eldritch god long forgotten. It plants pleasant thoughts in those around it, forcing them to help recruit for its cause, and converts bodies into more versions of itself when possible.
Devotion of His'tuun is a fight that could quickly spiral out of control. Alone, a Devotion is not a massive threat. However, if it is given some time, Devotion can recruit helpers through "Gaze of His'tuun", and convert addition Devotions with "One of Us" . Guards and adventuring parties will need to locate the creature quickly, else they'll quickly be overrun.
During combat, Devotion moves and fights erratically. It won't always make the most logical move, but it is a constant threat until defeated. Use Gaze of His'tuun to temporarily disable a creature, while utilizing the combination of the drag in potion of Tentacle with Bathe In Their Light to deal a constant damage to many people.
Devotion of His'tuun doesn't have much of a personality - it is a mindless being, hell bent on spreading the word of His'tuun through conversion either through One of Us or Gaze of His'tuun.
A person converted by Gaze of His'tuun will seem overtly giddy. They'll be inspired, will want to spread the word of His'tuun, but won't quite know who that is or why they love them so much. If questioned, the person converted won't really recall the past several minutes, and may not even remember being charmed by the Devotion. Any damage taken will bring them back down to reality and will cause them to remember everything they've forgotten.
With One of Us, the creature is fully converted and cannot be brought back. They will behave exactly like another Devotion of His'tuun.
Devotion of His'tuun was created into this plane. Willed, almost. His'tuun needs more - more worshippers, more recruits in its army, more power. Devotion will help with that, and will convert as many creatures as possible to the loving embrace of the Great One.
With Devotion of His'tuun, there are two main "phases" - the conversion phase and the combat phase.
During the conversion phase, there will be a lot of investigation of bizarre things, people speaking nonsense and talking of a creature they know nothing of. Maybe they'll come across the remnants of a creature that the Devotion fought. A nice eerie track that fits well with this is Blood Run from The Witcher 3: Blood and Wine.
Once the Devotion has been tracked down, an erratic combat will ensue. We still want that bizarre/horror vibe for this fight, and Tesham Mutna from The Witcher 3: Blood and Wine fits that bill very well.
