(This review is an expansion and edit of my review for v3.x of Strange Aeons. The original is still available here.)
Strange Aeons was originally released in 2015 as a three-episode megaWAD for the original Doom. Since then, Impie has added two additional episodes, bumping it up to a full 45 levels and including a patch so that it can be run in either Doom OR Doom II. As it requires ZDoom, it's something much more than a plain ol' Doom mapset; its almost exclusive use of textures from Chasm: The Rift, coupled with a few new weapons and a basically brand new bestiary, puts it right alongside the other "total conversions" of yore... and recent memory. It also has a plot, set up in the PDF and expanded in the end text of each episode, so you're free to get as invested as your imagination will allow.