Oathkeeper

Oathkeeper's true form in The Binding of Oathkeeper.

“

Oathkeeper wasn't named ironically; that thing is like the Patron Saint of Faustian Bargains.

— Johann Geist Jr.

Oathkeeper is a spirit residing in Lady Geist's arm that provides her with power and a youthful appearance in return for feeding on the life force on others.

Summary

Oathkeeper has been bound by a ward on Geist's arm, but the ward is nearing the point at which it can no longer contain Oathkeeper.

Oathkeeper seems to originate from a ripped page in the tome that Detective Abrams carries.

It is possible for Oathkeeper to be transferred to another person, this is shown in the novel where one of the options is giving him to Maximilian Fairfax. He is described as a "parasite" and can't survive for long without a host.

Lady Geist is performing the ritual in order to ask the Patron for a solution to Oathkeeper.