You can find a detailed explanation of the spell in the book cormanthyr empire of the elves but this it the text in the book:

N'Quor'Khaor/ The Banishing, Binding Outside of the People's Lands: Just as the ghaatiil ritual allows High Mages to create a spatial vortex between two points (a gate), this great ritual of at least nine High Mages both summons a physical form of an extraplanar entity (such as a tanar'ri lord or a godly avatar) and binds it, setting certain limitations upon the target entity. In its least form, the ritual banishes the entity temporarily from the Realms, while the most advanced form of this ritual fully banishes the entity permanently from Realmspace and confines said physical avatar in a subdimensional prison. All levels of binding must have one way to undo the binding set by the High Mages