Intensify Spells is a skill available in all spell schools. It requires 100 skill in the the school you want it in.

It increases the power of the spell in the schools you have it in, depending on skill level in said spell and the level in Intensify Spells

The power of the spell means that a damaging spell will do more damage, a heal more healing and a buff or debuff a bigger value of stats changed.
Your spells linked to Intensify Spells will have the same skill level as your Intensify Spells.
For the list of linked spells see Linked Spells

This effect stacks with Archmage.

Magic Schools
Regeneration MagicRaw MagicInfliction MagicAir MagicEarth MagicFire MagicWater MagicSpell ChantingArcane MagicWitchcraftNecromancy
Magic skills
Durable SpellsMana EfficiencyQuicken SpellsIntensify Spells
Magic Global Skills
Veteran MageMana ConservationArchmageReagent Conservation
Linked Spells