An epic is an exceptionally long poem that depicts a tale of epic proportions—usually about a famous or legendary hero's adventures and journeys. Epic conventions are the characteristics that the majority of epics have in common. Homer's Iliad is a great example of a classic epic, and many different epic conventions can be spotted throughout it.
In book 1 and book 2, perhaps the most notable epic convention is the fact that it begins in media res, or "in the middle of things." Book 1 finds Achilles nine years into the Trojan war. Although the plot is chronological, the reader learns about events before the Trojan war, such as Achilles's birth, throughout the Iliad through flashbacks.
Book 2 also opens in media res. However, it focuses on the events happening with the gods, while book I focuses on the humans on Earth. Zeus helps the Trojans by planting a dream in Nestor's mind and indirectly leading the Achaeans away from Troy.
Most epics use in media res as a way to delve right into the action from page one. They are generally extremely long poems, so utilizing in media res is a great tool for the poet to grab the reader's attention immediately while still being able to fill them in on the rising action later on.