A natural water feature in Western Commona, this the Mark of Ilios was a great lake, named by Common Humans after the nature god. The name likely developed in reference to the lake's preternaturally calm waters; even during periods of squall and turmoil in the surrounding region, the lake remained calm. This preternatural phenomena was thought to be from Ilios' divine influence, keeping the waters still and bountiful for the creatures who lived off them.
Box-like and estimated to be nearly 200 miles in length, the Mark of Ilios was by far the largest lake in the Head of the Greatwyrm save for its spiritual "brother", the Mark of Elteos.