Don't forget that the term Necromancer doesn't necessarily mean the same thing in-universe that it does for us.
He might be an Oathbreaker, so they could give him all sorts of bespoke powers. Not to mention the tadpole stuff.
Indeed, but... it confuses me personally that his new oath wouldn't be to his new deities. He's clearly a fanatic. This rejection could be an interesting personality detail, especially if he still values Shar above all. But, does he? Who's side is he on?
http://dnd5e.wikidot.com/paladin:oathbreaker
Some Paladin level details -- level 15 is important for the resistance to all non magical weapon damage. Level 15 boss?
Another point: undead in moonrise tower (trailer). Botched Experiments and all. If /he's/ a necromancer, he might be a good one.