[quote=@Sinerathin]
What type of Psyker were you planning on making when this thread starts?
[/quote]

Well I think a Primaris Psyker would be a little too much. As legendary as this Aphelius is I doubt he could court a Primaris into his plans. How about a Wyrdvane Psyker? one who can't control his power and as such hasn't been able to complete and/or attempt to become a Primaris. I'd prefer to stray away from a Sanctioned Pysker as they would be in active duty I would assume. Maybe, if the Wrydvane isn't possible simply a retired or at least some form of exile Psyker could be plausible. Either way I'm planning on making him rather unstable as you suggested. One of those people that nobody is quite sure how tainted they could be. 

EDIT: My other fancy was a Demonhunter.