Spell

Spells provide either passive effects or can be used in combat (And take up one of the spellcasters slots).

Spells that are used in combat are "bases" that can be modified by research. So you may have a spell that costs 2 Will and Gives every Ally a 2 Elemental Shield, a sage can modify the base depending on their magic traits - possibly lowering the Will cost, increasing the strength of the shield, or adding unrelated effects such as an offensive attack or a summon. The more lost arts and tomes the heroes recover the more their spells will benefit from it.

Other spells can aid strategically in hindering enemy movement on the map or altering the weather/terrain to your advantage, for instance you can rain out a plain to make the enemies cavalry much less practical.