Scion-tier 6th and last king of the Ur III dynasty (~2028-2004 BCE). Son of Šu-Sin; through paternal-Ninsun-line via Šulgi → Šu-Sin → Ibbi-Sin (1/8 divine, scion-tier per registry). Reign-period of progressive-collapse: Amorite migration through the failing Muriq-Tidnim wall; Elamite incursions; periphery-cities defections. The Ibbi-Sin Išbi-Erra Letter Collection (ETCSL 3.1.17-21, the "Royal Correspondence of the Kings of Ur") preserves the canonical primary-source dialogue with his rebellious general — one of the foundational documents of Mesopotamian political-historical literature. The fall of Ur c. 2004 BCE to the Elamite-Šimaški under Kindattu is the canonical-Sumerian-tradition catastrophe-event commemorated in the City Lamentations (Sumer-and-Ur, Ur, Nippur, Eridu, Uruk) — the foundational corpus of the Mesopotamian-tradition lamentation-genre that influenced the later Hebrew Bible Lamentations. Died in Elamite captivity at Anšan.