Eris looks like that because, as she hid for months in the tunnels of the Moon, she had to hide in the Shadows, as she was a Hunter, she quickly adapted to the environment there, resembling a Thrall's head, maybe when it glowed in the Dark, the Hive wouldn't attack her, I'm not sure. I just know she spent a lot of time learning their tricks to survive and probably the Darkness also slowly consumed her, making her resemble a being forged by Darkness itself.