When he does his Oroburos symbol is covered in blood, and in the same manner as Alphonse once did he regains his old memories as the old greed, more specifically that Wrath was the one who killed him several times as well as all his friends.
When he does, his Oroburos symbol is covered in blood; and in the same manner as Alphonse, he regains his old memories as the old Greed. Specifically, that Wrath was the one who had killed him several times as well as all of his friends.