The Empty Vessel (水の眷属 Mizu no Kenzoku, lit. Water Servant) is an enemy-only combat physical class that is exclusive to Fire Emblem Fates. The personal class of Garon in the Conquest route that is revealed after the demon masquerading in his likeness is unveiled by the throne of Castle Shirasagi, the Empty Vessel arms Axes as its sole weapon of choice.
In Chapter 15, Azura reveals to Corrin that in order to obtain peace to the world, they must kill Garon, which Corrin questions this motive, Azura reveals the truth with a one-time-use crystal ball that the real Garon had died long ago, and that the Garon they know is being possessed by a demon to spark the war between Nohr and Hoshido. Corrin agrees that Garon needs to be stopped and asks how they can show the truth to their siblings, Azura states that the only way for Corrin to prove this truth to their siblings is for "Garon" to sit on the magical throne of Hoshido's Castle Shirasagi, for whoever sits on the Hoshidan throne regains their true form and mind. This means Garon and his army will have to conquer Hoshido, win in Garon's name, in order to trick him into sitting on the throne, which will reveal his degenerated slime form, and then killing him to end the war.
After conquering Hoshido and killing Garon's loyalist, including Hans and Iago, Corrin finally tells the siblings that the war isn't over yet and they have to kill Garon to end it, telling them that Garon is no longer human and has become a monster and states that he cannot be saved, thus killing him is the only option.
In Chapter 27, Corrin and the Nohrians open the doors to the Castle Shirasagi's throne room, where they find Azura's claims to be correct as the throne has exposed Garon's true from as a slime monster, much to the siblings' horror. Garon curses Corrin for tricking him and attempts to kill everyone. Corrin attacks him, but their Grim Yato is unable to hurt him, and in retaliation, Garon launches a strong blow at them. In their disbelief, Elise, Camilla and Leo refuses to fight their father, but Xander is undaunted and calls out Garon and affirms that the monster is no longer his father, referring to his real father as doting and loving, stern, but fair, not using his strength against his children or starting a needless conquest of another kingdom. Xander convinces the siblings to put aside their disbelief and fight alongside Corrin to put an end to Garon. The Nohrian siblings all declare they will fight Garon alongside Corrin, and the proper fight begins. They manage to kill Garon, putting an end to his reign and deceitful crimes.