Grungni and Grimnir were both found by Sigmar whilst he was exploring the Iron Mountains. They were both imprisoned on the highest summit and where freed by Sigmar. Once freed Grungni pledged to settle his debt to Sigmar by offering to help create whatever he wished.
Grungni then went on to forge the Stormcast Eternals during the Age of Chaos. He does so no more, as tension rises between Sigmar and Grungni after Grimnir was killed settling his debt to Sigmar. Grungni is part of the accumulation of Duardin gods known as the ancestor gods.
They include Vhulkhaya lord of brew and gold, Grednir god of jewels and gems, Grimnir god of battle and grudge, Morgrim of engineering and thought and Gazul, king of the Duardin dead, Grednir of gem and artistry and Thungni of runesmithing.
Grungni is the Duardin god of mining and smithing.