Mercer was the captain of The Slumber , until his mind was seemingly shattered and taken over by the Herald of the Void , controlling his actions and leaving him in a catatonic state in the bridge.
 
There he remained until Lyraëlle snuck in an tried to get his ID tag. At which point he snapped and relived the trauma, causing the Herald to reappear. With his last shred of sanity however, he gave Lyraëlle the tag as well as his gun. After which, he actived a blast shield around the heart and the hearld and tried to buy precious moments for the rest of the party.
 
His fate was violent and quick it seems by the screams heard.