Helevorn was the deep, mountain-shaded lake in northern Thargelion at the foot of Mount Rerir. A stream flowed from its southwest corner into the Greater Gelion.
After the return of the Noldor to Middle-earth, the people of Caranthir dwelt about the lake. Its waters were defiled by Orcs during the Dagor Bragollach ("Battle of Sudden Flame"). It was destroyed in the War of Wrath along with Mount Rerir, though it partially survived.