What do you call a long serious poem that tells story about hero?

A long, serious poem that tells a story about a hero is called an epic poem.

Here are some examples of famous epic poems:

* The Iliad and The Odyssey by Homer

* Beowulf (Old English)

* The Song of Roland (French)

* The Divine Comedy by Dante Alighieri

* Paradise Lost by John Milton

* The Mahabharata and The Ramayana (Indian epics)

Let me know if you'd like to learn more about any of these epics!