The Epic of Gilgamesh, written in ancient Mesopotamia around 2100 BCE, tells the story of Gilgamesh, a king who sets out on a journey to find immortality after the death of his friend Enkidu. This quest leads him to the underworld, where he encounters the goddess Ishtar and the god Utnapishtim, who shares the secret of eternal life with him.
However, the descent into the underworld in Gilgamesh is not the central focus of the epic poem. Other poems from ancient Mesopotamia, like The Descent of Inanna, explicitly focus on the descent of a deity into the underworld.
So, while the Epic of Gilgamesh is the earliest epic to feature a descent into the underworld, it's not the first poem to focus solely on this theme.