"Okay, the fact that it's not them doing it does change things. In that case, I'd have them force-fed, find out who their orders came from, and if possible threaten them with it. That even leaves the vassal without a master.

Although, if fairy communication is as bad as you say, I might be able to get away with posting a fake name. If stealing their immortality is successful it'd probably kill them, and the perpetrator never being seen again is consistent with them running and never stopping."