What Babylonian creation epic begins with the words 'When on high'?

The Babylonian creation epic that begins “When on high” is Enuma Elish.

Enuma Elish takes its modern title from its opening Akkadian words, conventionally translated as “When on high.” The poem is written in cuneiform and is preserved on seven tablets, with surviving copies dating mainly to the first millennium BCE. Its narrative explains the origins of the gods, the ordering of the cosmos, and the rise of Marduk.

At the beginning, primordial freshwater Apsu and saltwater Tiamat mingle. Their offspring generate conflict, and after Apsu is killed, Tiamat becomes the great opponent of the younger gods. Marduk defeats her, forms the ordered world from her body, and becomes king of the gods.

The epic is especially associated with Babylon and with the New Year festival, when Marduk’s supremacy and the Babylonian king’s role in maintaining cosmic order were celebrated. Lugal-e is a Sumerian heroic poem, Descent of Ishtar concerns the underworld, and Atra-Hasis focuses on creation, catastrophe, and the flood.