Cloister is an epic level spell created by the wizard Dorukan. It is the ultimate abjuration spell; according to Celia, it blocks out teleportation, scrying, plane shifting, locating creatures and sending ethereal jaunts. Dorukan wore the focus for the spell as a headband.
Once someone inside the Cloister left it, the effects would still follow them temporarily (one week per caster level).
The spell has a specific exception for Summoning spells provided they were cast from inside the cloistered area, which allowed Dorukan to summon Lirian to his chambers and allowed Redcloak to summon devils. It also allowed Haley to accidentally summon Celia to Azure City with Roy's talisman after it was damaged by Tsukiko's Electric Orb spell.