The Iliad is a long epic poem written by Homer around the late ninth or early eigth century BCE. It is notable for being a source of Greek mythology, as well as being the oldest surviving text in all of western literature.
The Iliad tells the story of Achilles, and most of the Iliad takes place during the Trojan War.
You can read the full translated text of Homer’s Iliad here.
The Iliad by Homer