Here's the counterpart to my nurgle sorcerer. This sevrin loth model from forge world is sweet. I really like all of the detail on him and the casting pose. I swapped a black templar axe for him as well and did lightning like I have been for the rest of my models. The light blue was done to subdue the colour scheme to match the light white/cream of my space marines. I magnetised the backpack as well so it can be removed and swapped for a jump pack at a later date sould I want to even though currently I don't have one or see me wanting one.
Judging by the rules of space marines currently I'm not sure I'd often use a power armoured librarian as I much prefer a storm shield terminator version but I think using him as Sevrin loth would be really good. Running around with the full biomancy tree and 3 spells sounds awesome and I'll have to try him out.