Yes, EPOS is a Scrabble word!

Letter values in Scrabble US: E 1, P 3, O 1, S 1.

Is it a valid word?

EPOS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a body of poetry that conveys the traditions of a society by treating some epic theme
a long narrative poem telling of a hero's deeds

Also: epic poem, heroic poem.