Mana Conservation greatly decreases the Mana cost of all spells and stacks with Mana Efficiency for the schools that you have it Mana Efficiency in.

You need level 50 Intelligence to access this skill and costs 500Gold

Patch 2.2
Requirements changed from 75 Intelligence to 50.
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