A natural border between Commona and the Body of the Greatwyrm, this greatlake was named by the Common smallfolk after the nature god Elteos. The name likely developed in reference to the lake's turbulence; even when the oceans stilled and the Sea of Clouds calmed, the Mark stormed. This preternatural phenomena was thought to be Elteos' divine influence, inviting chaos and danger to the waters.
Unusual for a lake of its size, no permanent settlements lay along the Greatlake. Its constant storms made fishing difficult, sailing dangerous, and life generally miserable. It was also prone to swelling, destroying farmland and shelter, and those who would fish its waters found that many dangers lurked below the surface, as all sorts of monstrous creatures made the lake their home. Though the Mark of Elteos was incredibly dangerous to navigate for Commoners, even greater were the dangers beyond it. Few who crossed to the lands of the Giants returned unscathed, and few who sailed north to Sea of Clouds came back at all.
Estimated to be at least 600 miles in length and possibly extending further beneath the Sea of Clouds, the Mark of Elteos was by far the largest lake in Commona, dwarfing its spiritual "brother", the Mark of Ilios.