I'm currently working up an npc creature borrowed from Harry Connolly's "A Game of Cages" (Which I can heartily recommend to anyone who hasn't read it or it's prequel "Child of Fire").
The creature is the "Sapphire Dog", the McGuffin and supernatural villain of the book. An intelligent Lovecraftian being with a lamprey mouth and scaled polyped skin, that exudes a persistant aura of love, anyone without supernatural protection who sees it is effected and becomes obsessed with being the dog's sole provider and carer, to the extent of killing anyone they see as a threat to this goal, as soon as they leave the dog's presence the aura will dissipate and the victim will be able to act as normal.
The creature can feed those who touch it, permanantly draining them of emotional capacity until they become its devoted puppets (the book describes them as it's "Pets").

One of the secondary abilities the creature uses fairly often throughout the book is the ability assume the form of an ordinary looking (but still blue) dog.

I had considered some form of Glamor or illusion for this but wondered if this ability could be mimicked via the Human Form power (my only concern is that the power by it's very description allows the assumption of a mundane human form.)