The Abjurer attunement is designed to break existing dominion effects. Its deep dominion is the Dominion of Banishment. This can be used to banish summoned beings to their home plane and prevents their return, at least temporarily. This weakens the beings considerably. Additionally, Abjurers can reportedly temporarily stop attunement functions, an ability seemingly unique to the Abjurer attunement.
The Abjurer attunement is a restricted attunement, meaning it is usually obtained directly from a visage. Warren Constantine claims that the Spider Spire also gives restricted attunements, but whether or not this is true is unconfirmed.