The Coil River runs from the Throal Mountains through Lake Ban to the Aras Sea. It is one of the most prosperous tributaries of the Serpent River and along its shores settle more Name Givers than anywhere else in the waterway-system.

The northern portions of the Byrose and its tributaries spring from a series of tunnels and caves that are navigable up to the Kingdom of Throal, although access is forbidden to all vessels unless given express permission of King Varulus III.

Before the Scourge, the northern portions up to the tunnel system were deemed unnavigable; the most northern point reachable on the Coil River was Adipae Ford, a small settlement about fifty miles north of Lake Ban. The T'skrang Riverboat Captain Westhrall discovered a way through the rapids up to the Kingdom of Throal during the Theran War. Today the Adipae Rapids are so well mapped that few captains fear them and thus the Coil is navigable 250 more miles upstream to the small town of Darranis, from which a major caravan route leads to the city of Bartertown.

At Lake Ban, where the Floating City of T'skrang House V'strimon is located, it intersects with the Serpent River, before it continues on its way to the Aras Sea. The Coil’s southern reaches are lined with farming villages; the city of Axalalail is here located as well on the northern bank. Shortly before it enters the sea, the river divides into three branches which form the Coil Delta. Here the city of Urupa is located.