The most useful spell.

Crystal of Secrets. Thanks to the tactical gameplay and the AI's shortcomings, you can devise incredible scenarios on the map, much like in chess. Block a passage? Sure. Block an archer? Sure. Thanks to the crystal, you can eliminate absolutely any single stack of archers. And how did I defeat thousands of phoenixes with a thousand gremlins? It was all thanks to teleportation and crystals; by the end of that game, there were over 20 on the battlefield. This is the most useful spell from the summoning school. Chaos spells are useless when playing on huge maps; I never upgrade this school. Darkness. A top-tier school, thanks to mass slow, berserk, mind control, and vampirism (an absolute top spell). Recently, I used 16 red dragons to defeat 500+ dusk elves. It was all thanks to vampirism. A high-level stack under this spell is virtually invincible; there is only one countermeasure: Titans and their lightning. On huge maps, these are the most terrifying units. The school of light is useful for buffs and the "storm wind" ability. Therefore, the top 5 spells are: Crystal of Secrets, Vampirism, Berserk, Mind Control, and Mass Slow.