Wikipedia
Ištaran (also Gusilim) was the local deity of the city of Der, a Sumerian city state positioned east of the Tigris on the border between Sumer and Elam. His cult flourished from the Early Dynastic III Period until the Middle Babylonian Period, after which his name is no longer attested in the personal names of individuals. The beast and symbol of Ištaran, as frequently represented on kudurrus, is a snake (presumably representing Nirah, the snake god who acted as Ištaran's minister). The consort of Ištaran was known simply as Šarrat-Deri: "the queen of Der".
As early as the Early Dynastic period, Ištaran was being called upon as a god who might abjudicate in an inter-city border dispute between Umma and Lagaš. Scholars have suggested that his supposed effectiveness in this case might well stem from the border location of his own city, Der. His worship certainly spread beyond his own borders: perhaps in gratitude, Gudea, ruler of Lagaš, records his installation of a shrine to Ištaran in the great temple of Ningirsu at Girsu.