I tried to unlock the special ability for all the heroes I played: the knight, the demon, and the necromancer. In the end, I only succeeded with the necromancer – in the last mission, he reached the tree, gained level 30, and unlocked the Shriek of Despair. Without it, it would have been very difficult to defeat 1750 swordsmen of Godric.)) I had fewer skeletons – around 1200-1300. It's a pity that I didn't get the special ability with Agrael, although I had all the chances, but for some reason, instead of the last skill – Deadly Strike, which is necessary for the special ability, I started getting additional skills that I didn't need.
Now I'm wondering if it's worth leveling up Railagh's special ability.