Mana Shield

The complete inverse of Cast from Hit Points: having damage hit your Magic Points instead of your Hit Points. Unlike Cast from Hit Points, you generally don't see this come up as a plot point, as you can't exactly die from it or anything. Not to be confused with a barrier spell that uses magic points to increase your defenses.

Sometimes, this can turn a Squishy Wizard into quite the tank. Can become a Game Breaker by giving Nigh Invulnerability to the user. Especially if damage that would overkill the user's MP does not get converted into actual damage. On the other hand it will quickly turn a magic user into a sitting duck if the mana doesn't regenerate. It is also usually far more efficient to use your MP for healing spells rather than to absorb the damage directly.

Compare Mana Burn, which is where the attack is directed at the mana in the first place.

Examples of Mana Shield include:

Mages in World of Warcraft have the spell Mana Shield, which causes damage to briefly be dealt to your Mana instead of your Health.
- Of course, Warcraft III did this before with Mana Shield on the Naga Sea Witch.
  - Of course Diablo did it before that with the Mana Shield spell. All by Blizzard of course.
- In World of Warcraft Lady Deathwhisper casts one of these at the start of the fight, and doesn't drop it until her mana has been entirely depleted. Not that it stops her from casting spells after anyway.
Final Fantasy Tactics has the Time Mage ability "MP Switch" for this effect. Final Fantasy Tactics Advance and Final Fantasy Tactics A2 have the "Damage > MP" and "MP Shield" abilities respectively. The former didn't put any overflow damage into HP (Which made it incredibly useful for the main character to have for those 1-on-1 story fights), while the latter does, and since MP also starts at 0 in the second game, this skill is a really bad choice for classes that actually use MP.
- Particularly irritating opponents in the second game have both Damage > MP and Blood Price, the latter of which allows them to cast spells out of their HP. Particularly irritating ones can also heal themselves for more HP than it costs to cast the healing spell. Kill them quickly!
Diablo 2, the Sorceress has an Energy shield that diverts a proportion of damage to Magic points.
- ...but it's nowhere near as good as the one in the original Diablo, which completely diverted all damage to mana. Vitality is for the weak!
Maple Story, although it only absorbs up to 80% instead of full damage. Still very important for mages to master, as they would die to nearly anything in one hit at the higher levels without it.
In Super Robot Wars, barrier abilities reduce damage while draining energy.
The Magic Armor in The Legend of Zelda: Twilight Princess is a variation- it diverts damage to your money supply while also draining it at a steady pace to fuel its effects (as there is no Mana Meter in Twilight Princess).
There's the Vigor ability available to psionic characters in Dungeons & Dragons. It uses psionic powers in order to create temporary hit points, so its effect is similar.
- Similarly, elans can sacrifice power points to reduce or eliminate hit point damage once per round.
The Wraith Armor in Blood Omen: Legacy of Kain.
The Endure skill in Blue Dragon.
One of the many Geo Effects in Disgaea 3 swaps your HP/MP values, so that running out of MP can get you killed on these spots. Good for quickly wiping out powerful enemies with low MP; not so good for your Dumb Muscle, if you have any...
Technically the Neptune card from Castlevania: Circle of the Moon heals you when you would be damaged by the element in question, but the healing's so minor and the drain on your MP so major that it's really more like the attack just drains your MP.
- It's a miracle-worker, though, in the second playthrough's "Magician" mode, where you have very little defense and HP and a crap-ton of MP and intelligence (MP regen). Since you get all the cards at the beginning of the game in Magician mode, and you also have the Uranus/Unicorn combo to heal, plus almost every enemy in the game (including bosses) attack with an element, you become basically invincible provided you don't mind switching your cards whenever you have to deal with a different element, go on the attack, or restore accidental damage.
In the first Fable, this spell (dubbed "Physical Shield") is quite a Game Breaker. The game features a combat multiplier, which goes up as you're dealing damage to enemies and multiplies the experience you receive. Being hit resets it back to zero, but only if your hitpoints go down. If you absorb the damage into your mana, the multiplier stays. Using the mana shield constantly thus allows you to get the multiplier ridiculously high and level up much faster than you could without it.
Defense of the Ancients: Medusa has this skill
Heroes of Newerth: Electrician has a skill that reduces incoming damage by half at the cost of mana.
Dungeon Crawl has an amulet of guardian spirit that has this effect. It's new for ver 0.6, I think.
Magical Girl Lyrical Nanoha A's: The Battle of Aces uses this. Any attack that hits your barrier takes away from mana and being drained of all mana while guarding causes a guard crush with stun until the mana automatically regenerates.
An item in Order of Ecclesia causes damage to your heart supply (only used for limit breaks in this game) rather than your HP.
In the .hack GU Games, Doppelgangers are already tough as they are, what with being automatically 8 levels above the main character, and having an outrageous HP/SP regen points, and having a very damaging weapons equipped, complete with devastating effects thrown in for good measure. Come Volume 3 (At Walking Pace/Redemption), and the Doppelganger has the Mana Shield in full force, making fighting him an exercise in patience, frustration, and revival items, as, for a good amount of time, your attacks will do exactly 0 damage until you run its SP to 0. On the good side though, once you do, he won't be able to do Arts, so that's one less worry to deal with.
Oracle of Tao has a variant. Yazim Jianne, the hero that can use this can't actually use it to defend against attacks, but rather it heals him after the fact by shifting massive amounts of mana over to health.
Puzzle Quest and its sequel have skills for magic-user characters that let you take damage from one of your mana pools in place of your HP. There's usually an upkeep cost for using them, so the duration of the spell is typically how long you can keep chaining mana to keep the shield going.
In Midgard (back in 1971) wizards used up "endurance points" to cast spells and defended from spells cast by enemy wizards by spending the same points. See here.
Villains and Vigilantes has a mechanism called "rolling with the hit" in which a character can reduce HP damage by up to one-tenth of their current power point score -- essentially superhero mana -- by taking the damage to their power points instead.