Duelist’s Etiquette (WOTBS)

You touch the ground, and a 30-foot radius of the spell’s area is traced with a faintly glowing line of energy. The spell creates a subtle defensive barrier against magical attacks, causing all damage from spells and summoned creatures that originate in the area of effect to become nonlethal. A creature brought to 0 hit points from nonlethal spell damage is unconscious and stable. Whenever a creature enters the warded area or is present when the spell is cast, it must consciously choose to accept this restriction, or else the spell ends. If at any time a creature inside the area no longer wishes to abide by the wards, they can spend an action to end the spell.

This spell is primarily used to ensure that spell duels are not fatal, without forcing mages to hold back their strongest powers.