I wound up going the route of something being done to him rather than taken away from him. We've got a plot macguffin (fragments of the purple crystal from the City of Night) that's weakening the boundaries between Ningen-do and the Spirit Realms; I figure that's a good hook for the creation of Maigo no Musha, and so Emma-o required the NPC to consume a piece of the crystal, which will have the mechanical effect of making him vastly more vulnerable to spirits. (Basically Cursed by Maigo no Musha, except the realm doesn't exist yet.) Not sure yet how the PCs will create the realm, but at least I've got a foothold for that in place.

I do have to say, though, this was my favorite session to run in quite a while. One of the PCs had a way to step through into Meido, so she went with the NPC on his journey after death, helping protect him from the trials of along the way (drawn from Japanese Buddhist folklore), while the PCs back in Ningen-do came up with suitable offerings and prayers to various Fortunes who seemed like the right ones to advocate for the NPC as he faced each judgment. It ended up being a really great sequence, quite apart from the ongoing effects of what Emma-o did at the end.