Algarth's Embattlement (EFA - A)

Sometimes referred to as "the box of spells," this mighty magic stores spells you've previously cast, letting you release them via silent force of will alone

As part of casting Algarth's embattlement, you cast up to 6 additional spells of any level lower than 9th, each in the usual fashion, using the appropriate material components and spell slots. The slots are still used up, and remain used until you release the spell corresponding to the spell slot, and you can't prepare a different spell in that spell's place until you release it. In addition, your hit point total is reduced by 1d6 for each spell you cast into your embattlement, and you can't regain those hit points until you release the magic

You can release a stored spell by a silent act of will which requires no action. You must be conscious, and you must comply with any of the stored spell's requirements such as being able to see the target, but you needn't be able to speak, move, or have the necessary spell components

The stored spells cannot be released or dispelled except by you, and their latency persists even if you die. Once the duration expires, however, stored spells dissipate.