This is the mastery skill for spells of type “protection”.

The in game help reads:

Skill duration: random.
Type of skill : spell mastery (arcane skill).
Affecting stats: None.
It uses no endurance points.
Protection magic is one of the most useful types of magic known. Many
protective spells offer resistance or immunity to many hazards, some of
them fatal without such protection. Those who have studied and practiced
this art diligently are able to provide their spells more effectiveness,
reliabilty and duration. Many times there is need for only the best of
these magic users, due to the magnitude of danger involved in the current
crisis.