Water Dragon Possession: Black Marlin is a Teigu that takes the form of a ring. It was formerly owned by Liver and is currently owned by the Revolutionary Army.
Black Marlin was created using a body part of a dangerous aquatic beast that had the ability to control water. Some time before the start of the series, Liver got a hold of the Teigu and used it in many battles alongside his comrades. After his death, the Teigu was repossessed by the Revolutionary Army. During the rebellion against The Empire, one of higher ups notes to Honest that Black Marlin, along with Scream and Extase, is being used by revolutionaries against the Imperial army.