The Hero's Journey is a framework that scholar Joseph Campbell came up with that many myths and stories follow. Many storytellers and story-readers find it a useful way to look at tale. (That's actually putting it lightly. Some people are straight-up obsessed.) Chris Vogler adapted Campbell's 17 stages of a hero's journey, which many screenwriters use while making movies. Vogler condensed Campbell's 17 stages down to 12, which is what we're using. Check out a general explanation of the 12 stages.
The story of "Atreus and Thyestes" doesn't fit perfectly into the Hero's Journey structure, but we're giving it a shot. As the gross old saying goes, there's more than one way to skin a cat.
Atreus and Thyestes are good little boys living in the city of their father, Pelops.
Call To Adventure
Their mother, Hippodamia, asks them to kill their stepbrother, Chrysippus, because she doesn't like some other lady's kid around.
Refusal Of The Call
There's no refusal, Atreus and Thyestes jump right to it.
Meeting The Mentor
Well, we guess their mom is their mentor... their mentor in murder! Mwah ha ha!
Crossing The Threshold
After Atreus and Thyestes kill their brother, they run away from home fearing what their dad will do to them.
Tests, Allies, Enemies
They make friends with the rulers of Mycenae, and Atreus eventually becomes king. However, Thyestes makes an enemy of his brother by sleeping with his wife and trying to steal his throne.
Approach To The Inmost Cave
When Atreus invites his brother to a dinner party, Thyestes has no idea the sort of hell he's about to walk into.
Atreus serves Thyestes a dinner, which turns out to be Thyestes' own sons cooked and served on a platter for him.
The only reward here is Atreus's own grim satisfaction.
The Road Back
Thyestes is eventually dragged back to Mycenae, where Atreus plans to have Aegisthus, Thyestes' last living son, unknowingly kill his father.
That tables are turned when Thyestes and Aegisthus figure out each other's identity, and Aegisthus kills Atreus instead.
Return With The Elixir
There's no elixir here, just a long cycle of horror and bloodshed awaits the survivors and their descendants.