There have been discussions about certain overpowered spells and strategies (Impulsive+Fireball, Maul,...), but I haven't seen a thread picking up on the death spell "Graveseal". The spell itself is pretty killer, but that alone doesn't make it overpowered I guess, I actually like the spell. My problem stems from the fact that it is seriously overpowered on the cost/effect scale.
For the oblivious among us, the affected target suffers constant critical hits from all attacks unless it resists the attack. The spell costs mere 12 mana to cast (which is further reduced with all the mage traits) and has no further prerequisites considering the type of attack or other conditions that have to be met. In my experience, as soon as you get the spell early in game you have won, since your entire army crits down almost anything like there is no tomorrow.
I therefore suggest to increase the mana cost to represent its crazy usefulness (like 50 or even more) and/or make the spell only apply/work under certain conditions (increase the resist chance against it altogether, only death spell damage crits, each round there is an increasing chance that it wears off...etc.), something to that effect.