And also, a sworn enemy allows the hero to use a poisoned arrow, for example, along with a rain of arrows. Some mages might even be envious. If you find a Tome of Darkness (which is rare), then it's game over....3 Berserks = intense.

But that's not the case. Look, a poisoned rain of arrows isn't that great because you might have 4 units as enemies - let's say it's a Death Knight, an Archlich, a Zombie, and a Skeleton Archer - and casting a poisoned rain of arrows will waste mana on the archers and zombies. If all 4 are high-level, then yes, it's effective. But on low-level enemies, it's better to cast a cheaper curse.

P.S. Try casting a triple Berserk))) You'll see what happens...