Version 2.1 of the Epsilon edition of the Quake expansion called "The Myths".
Soundtrack by Ochre (ochremusic.com)
This excellent expansion/6-map pack is brought to us by the talented vIE888, who has graciously allowed me to adapt it to work with the epsilon packs.
It's a very difficult, tricky and puzzle-y sort of set, bound to test your brains.
Even the first level is pretty tricky to get through (hint: you need TWO tridents, not one)!
Another hint: the end sequence text is a joke (as explained on the moddb page, there is no "commercial version"...).