Innate Spellcaster(X) means that a mage will be able to cast X more spells per round in addition to his normal actions. Also, he will disregard spell casting times, effectively casting all spells at the same 100% rate. On top of that, innate spellcasters cannot be disrupted and prevented from casting spells by being attacked in melee.

Innate spellcasters who are masters in a communion pass on double the fatigue to the communion slaves. An innate spellcaster can never become a communion slave.