If you talk to the Gray Prince before this point, he will ask you to investigate his past for him, starting a sidequest called “The Origin of the Gray Prince”. He will be so demoralized by what you tell him that when the time comes for you to challenge him, he will allow you to kill him without a fight. If you choose not to do this sidequest, be prepared for a challenge. He’s quite strong with several powerful attacks, so bring a number of healing potions with you. To fight the Gray Prince you must talk to Ysabel Andronicus, the Battle Matron. You can find her at the opposite end of the room as Owyn in the Bloodworks. After you beat him, take his Raiment of Valor off his body and return it to Ysabel. The achievement will unlock as soon as the conversation with her ends.