The Myrfrus River, also known as the Deeprun, is the only major body of water in Iobaria's Dvezda Marches, and protects the erstwhile nation's traditional eastern borders. The Deeprun crashes over numerous rapids and waterfalls in its run through the Deeprun Crevasse, with 50 of these to be found between Daruthrost and the river's mouth at the Castrovin Sea.
The Myrfrus flows through a portion of the Hoofwood where the woods spill over the cliffs and into the Deeprun Crevasse 30 miles south of Artrosa. Leaving the woods, the river then flows over rugged cliffs and enters a wide plain where it spreads out to form Lake Pirstjoi, from which the island of Zvartjan rises. South of the lake, the Deeprun passes under the ruined cyclopean bridge known as Daruthrost and leaves the Crevasse, heading south to empty into the Castrovin Sea at Mirnbay, Iobaria's largest city. The rugged, largely barren geography of the Crevasse alongside the numerous creatures that lurk in the Deeprun's waters and lair upon its shores make the Myrfrus an unsafe trading route, despite its connections with the Lake of Mists and Veils via the Ardshrod River.