The most useful spell.

What about the Wall of Swords?

Added after 56 seconds
Is it as powerful as Dispel, the Doom spell? What about regeneration and resurrection?

The Wall of Swords requires a summoning skill, while the Crystal is always available. Plus, it consumes less mana, which is relevant when you place 20 of them. Also, this tactical defense is usually aimed at large units; you don't need to completely block everything, especially if the terrain is complex.

Dispel? We need to cast spells, not dispel them. It should be noted that I'm not playing against players, but against the AI on huge maps at the "Hero" difficulty, which I've further enhanced in the editor with powerful stacks. For example, a million peasants, 10,000 level 7 units, 20 level 6 units, etc. Who wins is a matter of luck, but the goal is to eliminate everyone. Of course, it's impossible to defeat the army head-on, so strategy comes into play, especially when facing undead, against which Berserk is ineffective, and mind control and vampirism are useless.

I'm not sure what "Doom" is; it might be a nuance of localization. Is it the "Darkness Weakness" skill? It's a useless thing.

Resurrection is weak, suitable for battles with weak units, not the main army. Regeneration is also a good thing. But vampirism is better