I think both spells should be in the mage tree and should be changed:

Mana Shield: Your mana absorbs 50 % of the damage

Mana Blast: Inflicts 10 + 3 per shard of any type points of damage

Except they completely destroy any semblance of balance.  Good thing they're only available as a quest reward (so far as I know).

Maybe he has so much mana because he started saving it for precisely this reason?

I almost never use the permanent stat/exp boost spells now with how expensive they are, prefer to use the mana on Falling Star and take care of stragglers that way, with a side of Freeze/Tremor/Pillar of Flame. If you could hotkey the boost spells maybe I'd use them, but otherwise going through the spellbook to cast them over and over gets very tiring.

I have a lot of mana because I don't bother with permanent boost spells.  In fact I pretty much only use spells in tactical combat or to buff units with Evade and so on (which are not permanent).

Also like I said, slow pacing means I can get into hundreds of turns without getting close to victory.

I think it's a double fight (like the Arena, though only two fights) against stone creatures in the temple.

You have to have a very large amount of mana to get a big benefit, and if you're getting smacked for 100+ damage each round, even Mana Shield will run out.

The spells are amazingly powerful if you have a large amount of mana.

The issue I have with them is that anyone can use the spells.  It seems odd to have my henchmen casting mana blast, but I've done it one game.

I like the mana blast spell on the sword better than as a quest reward.  Had it in a different game & it seemed less OP.

Oh man, I got these in one game... Put Mana Shield on my Sovereign (with Procipinee's Crown, so no upkeep!) and went nuts.... also destroyed Morian with two castings on Mana Blast while the rest of my army dismantled his support. So fun = P

Also, as to why stockpile that much mana? Repeatedly research Refined Arcana. Before too long, you've got several thousand mana stockpiled, and can soon use exp boost spell once a turn and not have your pool decrease at all = P

There's 1 problem with it as well, I noticed in my game that my henchmen all got the mana blast spell, and were doing 500 damage a hit with it, basically 1 shotting dragons, due to my enormous mana reserves.

IMO mana blast should be reserved for the sov, henchmen shouldn't get it nor other champs.