I had a lot of fun with NovelAI. I believe at the time it was using GPT2, and I loaded in fine tuned models for the canon of choice I wanted to experience (trained on fanfic, and things of that sort).
Spellbound is an instruct model to NovelAI's completion model: you enter commands which in turn dictate what happens to your character, then the AI models how others would react to you
How does spellbound work?