Oathkeeper

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. It has been bound by a ward on her arm however 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

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