Joseph Campbell condensed his understanding of story into a theory he called Monomyth, also known as The Hero's Journey or The Hero Cycle. His simple description of the Monomyth is as follows:
A hero ventures forth from the world of common day into a region of supernatural wonder: fabulous forces are there encountered and a decisive victory is won: the hero comes back from this mysterious adventure with the power to bestow boons on his fellow man.
Homer's Iliad is in itself not a Monomythic story -- it involves only a few weeks of the Trojan War and has no explicit hero. However, it is possible to examine the story from the point of view of a specific character and from there compare it to the Monomyth.
Achilles is usually considered to be the protagonist of the Iliad, as his actions spur most of the events along. His withdrawal from battle at the beginning is the action that causes the tide of battle to turn, and when he returns at the end, routing the Trojans in madness and rage, the story is more-or-less finished.
In the context of the Monomyth, therefore, Achilles has "ventured forth" from his "common-day world" -- his life based in battle -- and into "a region of supernatural wonder," deliberate pacifism during which he confers with the gods. This is unusual enough for Achilles to be considered rising action; he finds it difficult day-by-day to stand back and watch the battle without participating. As events unfold and a friend is killed in Achilles's armor, he returns to action and takes on a "fabulous force" of Trojans, who had gained the upper hand. His "decisive victory" is the killing of Hector, son of King Priam and the Trojan Army's greatest warrior. At the end of the tale, Achilles presides over a day of funeral games and literally "bestows boons on his fellow man" by awarding prizes.
This is one simple interpretation of the Iliad as Monomyth. Since there are so many characters and so many events, others could be drawn as easily.