Source: Lords of Madness
You can cast spells that do additional damage to aberrations.
Benefit: When you cast a spell that deals damage to an aberration, you deal an extra 2d6 points of damage.
A single spell can never gain this extra damage more than once per casting. For instance, a fireball deals the extra damage to all aberrations in the area it affects. However, if a 3rd-level wizard casts magic missile and produces two missiles, only one of them (of the wizard's choice) gains the extra damage, whether or not both missiles are directed at the same aberration or two different aberrations. If a spell deals damage for more than 1 round, it deals this extra damage in each round.
Scrolls scribed by you do not gain any benefit from Aberration Banemagic. Scrolls activated by you also gain no benefit from Aberration Banemagic. The same is true for most other magic items, such as wands and potions. However, staffs activated by you use not only your caster level but also gain the benefit of this feat, if applicable.
If the spell allows a saving throw, the DC of the saving throw increases by 2. The additional damage dealt by this spell is either halved on a successful saving throw or negated entirely, depending on how the regular damage dealt by the spell is affected by a successful saving throw.