Knowledge skill
The Knowledge skill (or stat) determines the maximum amount of Spell Points a hero may have. This is usually 10 times the hero's knowledge stat, but can be increased by the Intelligence skill. Visiting a Mana Vortex in a Dungeon or a Magic Spring can temporarily increase the amount of Spell Points over the max.
Note: Normally, all heroes start with at least 1 point in this skill. However, a mapmaker might set the starting skill to 0. In that case, the hero will look and function as if he had 1 point, but when he receives his first point, it will not have any effect.