Borg

Bolg (防壁魔法, Borugu) is said to be the proof of a Magician. It is a defensive ability. It blocks out attacks with evil intentions. It also blocks out most physical attacks and magical attacks to some degree. It's strength, however, depends on the person. According to Professor Myers, if the Bolg is attacked by immense physical strength, then it can be crushed/destroyed.

In the case of the Magi it seems that you are unable to dent their Bolg unless you use a Metal Vessel or a Household Vessel.

Trivia

 * Ugo is the only person capable of crushing a Bolg with physical strength to date.
 * The Magic Ugo is also able capable of destroying Bolg with physical strength.
 * When translated from kanji, Bolg (防壁魔法) means Magic Barrier.
 * The name of this ability is a corruption of the Arabic word بُرج Burj, which simply means "Tower" or "Citadel".