Well, there are a few things that should be done to address this IMO, nerfing the scaling of spells in general when there are only a few select spells that are OP to an extreme when stacking magic damage + spell mastery is unnecessary although would work.

1) Easiest way to fix this would be to replace spells that do raw damage to entire armies with strategic spells or tactical utility spells/debuffs.

2) Remove ability to convert shards OR grant it for all other magic schools. IMO all mages should be able to specialise in their chosen element so I would personally go for the latter.

3) Make it so all elements have spells worth specializing for, as it stands at the moment the only elements that would be worth stacking shards for is death and fire from what I can see.

Agreed, buffing magic resist and other elements would be ideal.

edit: Reasoning is armies that stack magic resist should be _good_ against mages not just get wrecked slightly slower. Also there should be other options for pure mage chars aside from death/fire.

Agree entirely with the OP and have made similar posts before. In that list of spells you should also include Wither: with enough death shards you can reduce pretty much any trained unit stack down to zero attack and therefore its possible that a single spell can effectively take out the entire stack. I've used this tactic to win on Insane difficulty levels.

There are two separate but linked issues here:

1. The convert shard ability is too strong. It allows you to 'break' the game by boosting the mentioned spells to extreme levels. If you applied this ability to other magic schools you'd get similar game breaking effects (eg dodge bonuses from evade that make units unhittable). This ability should either be removed or have a reduced effect, eg you need to convert three shards to gain one death shard power. Then there's a real trade-off between conversion and keeping the shard, at the moment it's a no brainer to convert.

2. Spells like Dirge of Ceresa and Contagion are over-powered. Doing substantial damage to every unit, every round for 10 turns, means they are so much more effective than any other damaging spell. A fireball will basically do 3 times the damage of a contagion spell to each unit. However, you're lucky to get three or four units with a fireball, whereas contagion will get all 9 and then damage them for up to ten rounds. Again, it's a no brainer to use contagion. I think dirge and contagion should be further reduced in the damage they do.