Personally, I wouldn't suggest giving your character a mental illness or any disorder of that sort. It feels a little too arbitrary, like you'd be putting it in just to try to make your character more 'interesting' and 'unique'. However, as Hanged said, if you feel as if you can pull it off reasonably well, please do go ahead. As you'd also be doing research prior to and in parallel with this character change, it seems you're fairly serious about it.

tl;dr tentative go ahead

[quote=TheHangedMan]Also, 15 is a bit young for even intermediate level Necromancy, even assuming he began practicing when he was still young (which is something that borders on illegal mind you) Either you lower his skill level or up his age and seniority, to explain his relative prowess.
[/quote]

Might I add that adding more "conditions" to the efficacy of the Necromancy would be of help, too. Things like "he'd only be able to be this good when ____ happens/____ is present". Uh... In general though, Hanged, perhaps the line of thinking that "geniuses can happen" may be appropriate. Seb might just have had the extraordinary luck of just... grasping such a complex spellcraft.

Perhaps he's a savant, though I doubt it. Athan did mention that he's not very good when he doesn't have his charms, so I'm guessing that without those symbolic/magical items, he's on the novice or lower-intermediate tier.