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In my opinion, the magic schools don't have enough spells. I suggest creating new ones for each school.
For example, for Dark Magic, a level 3 spell: "Sense of Death" – for each creature killed in the squad, damage is dealt equal to 1, 2, 3, or 4, depending on the magic skill level.
For Chaos Magic, a level 4 spell: "Thunderstorm" – deals damage in a 4x4 area, with the strength and duration depending on the magic skill. Damage is dealt before the hero's turn begins.
For Summoning Magic, a level 2 spell: "Quicksand" – the event has the same effects as falling into a trap. A 5x5 area is created on the battlefield with 1, 2, 3, or 4 quicksand areas (again, depending on the magic skill level).
For Light Magic, a level 3 spell: "Sacred Wind" – adds 1, 2, 3, or 4 to the speed. Also depends on the magic skill level. :D