A legendary being of great power, Mercerius is reputed to have existed before the Rivening. He is neither Ysar nor Aedonii, but something older and, apparently, more powerful. Ancient texts, both pre- and post- Rivening, indicate that the being is a great source of knowledge—if one can find him and approach him. His stronghold lies somewhere beyond the western edge of the Ridgepeaks, but the actual location lies unknown.

According to the texts, Mercerius is known to be a capricious host—his whims, everchanging. He often does not welcome visitors that come seeking what he can give and does not give his knowledge without trial.