I think the "Dirge of Ceresa" is far too overpowered. I can kill with this spell any army. I am droped behind in the tech tree, I havent got a decent army, I have only one deathshard, but I can devastate any army of the other faction. It deals poison damage, so it doesn't kill a lot of monster, but the armies of other factions have no chance.
This spell couldn't be accesible in early game. But see:
the spell: All enemy unit take 6 (+3 per DS) poison damage PER MEMBER an take 3 poison damage (+3 per DS) per turn FOR 10 TURNS.
Ceresa is on the 8. level, with 4. level death magic, and I have 1 death shard.
The armies (lots of army and 4 heroes on the other side):

Before my moves:

The first effect (I used only the "Dirge of Ceresa" spell (and a slow+web) and moved back my units. I haven't got archers and I didn't used my other hero.):

After two "rounds" the enemy army is nowhere. But I didn't do anything. They moved and died of poison:

From here the golem wasn't a problem. In my army nobody died:

It was fun, really . . . but