A Contingency spell allows a wizard to store a spell(s) that will activate once the condition specified at casting occurs. The uses for this spell are limited only by the imagination of the wizard.
Upon casting this spell, the wizard is allowed to choose a spell out of his repertoire of known spells. One spell level every 3 levels of the caster is allowed––up to 6th-level spells at 18th level. Once the spell is chosen, the caster may choose a condition of some kind from the list provided. The starting condition can be anything from reaching 10% of his Hit Points to an enemy being sighted. Once this starting condition is satisfied, the spell that was chosen will be automatically cast.
For example, a 12th-level wizard could place the spell Stoneskin in a contingency with the starting conditions being "the wizard reaches 50% of his Hirt Points." Once the wizard is reduced to 50% of his Hit Points or below, Stoneskin would be immediately cast upon him. This requires no action be the wizard, but happens automatically and instantaneously.
A limitation of this spell is that the target of the spell placed in contingency must be the caster: Therefore, it may not be an offensive spell, nor may it target any other party members.
Acceptable conditions are:
Wizard is hit for damage
Wizard sees enemy
Wizard's Hit Points are at 50%
Wizard's Hit Points are at 25%
Wizard's Hit Points are at 10%
Wizard is helpless (i.e., charmed, dominated, confused, stunned, asleep, etc.)
Wizard has been poisoned