Mana Shield

Mana Shield is a self-cast mage spell that shields the caster from all damage. It can be a mage's best friend or worst killer. When this shield up, physical and magical damage will be absorbed at the cost of 1.5 mana per point of damage. The shield collapses after absorbing a set amount or 1 minute passes, whichever comes first.

Talent improvements
The Arcane Shielding talent will reduce the amount of mana consumed per point of damage up to a 33% reduction at max rank. This makes the damage to mana ratio 1 to 1.

The Incanter's Absorption talent increases your spell power by 15% of of the damage absorbed.

Tips and Tactics
After it absorbs the given damage (consuming 1.5 times that amount of mana) it will collapse, but no cooldown means you can recast immediately, or even while it's still partially up to "refresh" the absorption pool.

Don't be fooled by the cheap casting cost. Mana Shield can drain your mana deceptively fast. Rank 3, for instance, will leave you (80 + (1.5 x 300)) 530 mana poorer in exchange for a measly 300 health. Use wisely.

The Arcane Shielding talent can make stomaching this spell somewhat easier - with all ranks of the talent active the ratio on the Shield is reduced to a much more manageable 1:1 Mana to Health trade-off.

The Incanter's Absorption talent however can make the mana used up worth the trade-off. 15% of the damage absorbed per hit will be added to your spell power. This effect lasts for 10 seconds but, it stacks on top of prior hits while refreshing the buff timer. This can make you keep the buff for the entire duration of the fight (as long as you get hit every 10 seconds) and the longer the fight goes, the more power you gain (up to 5% of your max health). A hit large enough to remove the Mana Shield in one hit (at level 80 with 1500 spell power) will increase Spell Power by 381 for 10 seconds.

Level 80 Mage with 1500 spell power using Ice barrier with Mana Shield:

Total Absorbed: 7046

Total Mana Used(to cast spells): 1046

Total Mana Used(by Mana Shield): 3807 (untalented) / 2538 (talented)

Total Mana Used: 4853 (untalented) / 3584 (talented)