Not sure I know the excact one you have in mind, but these might be similar.

any of the albums by Imperium. https://imperiumdeathmetal.bandcamp.com. they are all focused on Greek/Roman stories; on the death metal side. 
For Glory by Spartan, but its an EP not a 2 hour album https://www.youtube.com/playlist?list=PL3AA25B958A07D45B. This is on the death metal side
The Odyssey by Symphony X https://www.youtube.com/playlist?list=OLAK5uy_nGKw74O6hiUmYcUmVUP3evOUaYPK5_fS4. This is on the classic metal side
Shogun by Trivium, https://www.youtube.com/playlist?list=PLZ9DoO2uX9wWGek6MxYI1YJSwbZ8D73xs. Not entirely focused on Greek mythology but this has quite a few songs on it along with some nip focused ones. This is more metalcore/thrash inspired. 

Theres also one-off songs by bands like Iron Maiden, Sabaton, The Raven Age, etc. but i m not sure any of them have a single album focused on this