Hey thanks for playing and the great feedback! That's a good point that the bosses should telegraph more and I think that will fix some of the bosses being too difficult.
To clear up the seals mechanic, that probably should have been in a tutorial but we ran out of time. the seals have 3 possible types: passives, thresholds, and rotations. Passives just give passive stats, thresholds give some kind of bonus if a condition is met (like giving the player more ranged attack speed if they are above 80% health), and the rotations do something every time they rotate (could be temporary stats or shooting a projectile). Again this totally should have been explained somewhere so thanks for pointing that out!