Evaluating Intelligence affects the spell damage of various Magery spells (similar to how Tactics increases damage dealt in melee/ranged combat), but only direct damage spells such as Energy Bolt, Meteor Swarm and Fireball. It does not increase the strength of healing or poison, or indirect spells that cause damage like Summon Creature. It does not increase the severity of a condition like Paralyze or Protection. It can also be used to determine the target's overall Mana and Intelligence.
Evaluating Intelligence affects how much damage your spells do against your enemies. The damage bonus is multiplied by the spell's damage. This damage is based upon your target's Resisting Spells skill level as follows:
If the target's Resisting Spells is less than the attacker's Evaluating Intelligence:
Damage Bonus = 1 + ((Attacker Evaluating Intelligence - Target Resisting Spells) / 500)
If the target's Resisting Spells is greater than the attacker's Evaluating Intelligence:
Damage Bonus = 1 + ((Attacker Evaluating Intelligence - Target Resisting Spells) / 200)
Evaluating Intelligence also offers a chance to strike a critical hit with spells in PvE. With 60+ skill in Magery and Evaluating Intelligence, there is a maximum 15% chance to score a critical hit at GM, scaling based on both of these skills.
With 80 Anatomy and Evaluating Intelligence, there is a chance to dodge an attack and thus prevent spell interruption, similar to the effect provided by Wrestling. This provides more interesting builds for Mages.