The epic conventions in the Iliad include the story beginning in the middle of the action, the evocation of the Muse and the declaration of the theme of the story in the opening lines. The plot of the Iliad begins after 10 years of the Trojan War have passed.
The name of the epic convention or literary device by which the story begins during the action is "in medias res," which in Latin means "in the middle of things." The start of the action in an epic comes after the evocation of the Muse, who is the goddess of poetry.
In the Iliad, Homer evokes the Muse with the lines "Sing it now, goddess, sing through me the deadly rage that caused the Achaeans such grief." In ancient Greece, the epic convention of evocation was the means by which the speaker of the story channeled the divine inspiration of the Muse. The speaker of an epic poem was believed to be the vessel for the divine inspiration and therefore warranted the audience's attention.
The theme of the story is then presented within the evocation. In the Iliad, the theme of the story is presented in the opening lines as Homer describes "the rage of Achilles." This epic convention told the audience what the story was going to be about. By making the theme obvious in the beginning of the epic, the audience could then fully appreciate the story itself as it unfolded.