How much mana does it cost to use? It should be high. If it cost as much as the damage done, then you would deplete fast. With two heroes and 2000 mana, you would spend 380 mana on two uses of the spell.

I think it might also be your playstyle or settings, cause I dont always have too much mana. I like flame dart and my hero typically casts it twice in the start of a battle. Thats 20 mana right there. 

This seems OP when most other spells are doing 10 dmg.

Way too OP because in my games I have way too much mana gathered than that I'm spending (maybe i'm doing it wrong lol)

But still, it's too much dmg for that cost!

Well, more importian is how do you get this mana blast? Of course it's might from quest reward, but anything, it's very rare, in face, many of I play, I never got them, I don't think it's overpower at all, since I never got them in my many play and quest, but I think it's quest don't respawn in game anyhow.

Manablast is underpowered as it not comparable to high end late game melee damage. It takes a considerable amount of mana to do damage that is roughly equivalent to high end regular hits (not criticals which can do mind boggling amounts). After a few uses, it becomes sub-par as your mana drops rather fast by using it.

Not to mention that is a quest spell that needs to be found...

We need a spell that is generally more powerful (like the previous arcane arrow or blast or what not) that does a fixed damage, based on your intelligence (or now, spell mastery for example).

Apparently, the devs can't be bothered to implement scaling mana costs.  Big ol' whammie!