Here's the synopsis:
This story takes place in Wraiths. At the end of chapter 12 after Seel saw Pellaz in his re-generation tank, Thiede is very specific with Seel and after telling him he's going to move to Immanion within a month, says, "You will be pleased to know I've allocated a sedu to you. Your training in controlling it begins in two days' time. I'll send a teacher to Saltrock with the animal." Seel isn't particularly grateful.
And here is the fic. (Adult, with warnings for threesomes, and Wraeththu bits)