Sequester thoughts allows you to erase a creature’s memory of either an event lasting not more than 1 minute per caster level or all of its knowledge about a single topic (using the GM’s discretion as to what constitutes a single topic). For example, you could erase a single battle from a creature’s memory, or all knowledge of a plot to assassinate a king. The memories you remove are stored within the gem used at the time of casting. If the gem is shattered, the memories return to the creature as long as the two are within 30 feet of each other. Once sequester thoughts has been cast, the spell remains active on the gem and can be dispelled (which shatters it). No portion of the spell remains active on the target creature, and the target does not radiate magic as a consequence of the spell, nor can its memories be returned by dispelling the creature or subjecting it to antimagic. If the gem is shattered or dispelled out of range from the creature, the thoughts sequestered within are forever lost save by the use of wish, miracle, or the like.
Sequester thoughts protects against detect thoughts, zone of truth, discern lies, and similar spells where the memories removed are concerned, though careful questioning may reveal the gaps in the creature’s memory, or that it has been affected by the spell. Note that the creature itself does not remember any details of what memories were removed until the gem is broken.