The Atronach Stone (Skyrim)

Effect
The Atronach Stone fortifies magicka by 50 points and grants 50% spell absorption. However, it causes magicka regeneration to decrease by 50%.

Location
The stone is located between Windhelm and Riften, in the marshland, just outside the mountain range between the two locations. Near the Eldergleam Sanctuary and Darkwater Crossing, the stone is by the river to the East.

Strategy
The magicka regeneration decrease may appear to be a disadvantage at first, but when combined with the Recovery perks, the drawbacks can be negated, with the added benefit of an extra 50 magicka points and 50% spell absorption.

When combined with the Magic Resistance and the Atronach perks from Alteration and the Elemental Protection perk from Block, One can build a character that is highly resilient to magic. As a Breton, the Dragonskin power will stack with the Atronach Stone, making you immune to magic for the duration of Dragonskin.

See the Spell Absorption page for more information on obtaining 100% spell absorption.

Also given the way the magic regeneration system works, with the more magicka a character has, the more they gain per second. So with a 50% reduction to magic regeneration, the two ways to counteract this effect is to either get another 50% regeneration effect OR double the magicka capacity.

Bugs

 * Unfortunately for Conjuration mages, the game treats summoning as a spell being cast on yourself, thus the 50% spell absorbtion chance kicks in making your summons 50% likely to not occur. PC Users can avoid this bug by installing the "All in 1 Spell Fixes Mod"
 * As a bonus, when a summoning spell gets negated the player will "absorb" the magicka, this can be used to restore a low magicka level. However, since 50% of the time the spell still works normally, it can take a few tries and you also must have enough magicka to actually perform the spell.
 * Interestingly enough, when using the Dragon Shout Summon Durnehviir, should it fail the player will regenerate all their magicka, due to the game treating the Shout as a Conjuration Spell.
 * You can combine the magicka absorbtion with the shielding circle spell for infinite magicka while inside the circle.
 * The 'absorb' animation used for showing that an enemy's spell is being absorbed can also play incorrectly for non-magical attacks. This has been observed with everything from non-enchanted swords to a wolf's bite. The reason this occurs is because wolf/bear attacks have a chance to inflict a disease such as Rockjoint, so their attacks have a magical component (each time the animal attacks it casts a spell on you with a small chance to inflict a disease). Thus, the Atronach Stone reduces the chance for you to receive a disease from these animals by 50%. (All platforms)
 * The spell absorbtion effect may cause problems during Brelyna's Practice. In the quest a spell is cast on the Dragonborn, however if the spell is absorbed then the quest will not progress any further.