Mana Efficiency is a skill available in all spell schools. It requires 50 skill in the the school you want it in.

It will decrease the amount of mana your spells in the schools you have it in, depending on the skill level of Mana Efficiency in said school.
Your spells linked to Mana Efficiency will have the same skill level as your Mana Efficiency.
For the list of linked spells see Linked Spells

The effect of Mana Efficiency stacks with Mana Conservation.

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