
UPDATE NOTE: Shadow Warrior's "spell absorption" bug has been corrected by the latest version of the USKP, as of the latest release. Sneak's Shadow Warrior perk can also be absorbed. In that case nothing happens - the summoned creature does not appear, but you don't lose any Magicka either (because the Magicka spent is immediately refunded to you). There exists a bug with Magic Absorption where you can absorb your own summoning spell.You can absorb effects from weapon enchantments, poisons, diseases and even damage from being on fire. Magic Absorption works not only with all spells, but also with just about any effect that isn't physical damage.
So if you activated the Atronach Stone and an enemy mage hit you with a Firebolt, you have 50% chance to negate all damage from the Firebolt and gain 41 Magicka.
Magic Absorption is a random chance to completely nullify effects of a spell and, if the spell had a Magicka cost, add the casting cost of a spell to your Magicka. It is calculated before Magic Resistance. Magic Absorption is separate from Magic Resistance. Magic Absorption is a rare and more complicated effect, obtainable in only three ways in the game: The Atronach Stone (50%), Atronach perk from Alteration skill tree (30%) and Breton's Dragonskin racial ability (50% for 60 seconds, once/day). I randomly came across this description on the Magic Overview page: Seems it should be a page especially when considering people may look it up to see if it is a "percent of effect" or a "percent chance" effect. When referenced on other pages, other effects receive their own links, but this effect will be clearly lacking a link. I've run across references to this effect several times, and there is no page or redirect for it. 18 A Really Easy Fix for Spell Absorption of Conjuration Spells (Using the CK). 16 Conjuration Staves and Spell Absorbtion. 12 Spell absorption on Followers/Summons.
11 Dragonborn DLC 100% spell absorption.10 Testing done with version 1.7.7.0.6 shows a 85% cap for spell absorption.7 List of spells that are messed up by Spell Absorbtion perks/stones/enchantments?.6 Regarding Vampirism/Necromage and Spell Absorption.