Marathon: Red addon

An intense, 32-level near-total conversion. No solo scenario has scared me like this since Evil... this has a very Aliens-like feel to it. Add to that the incredible physics tricks performed... no other scenario has let you change sides completely, halfway through a scenario. The storyline is confusing, at first... but it's meant to be. (It becomes clearer near the end.) There are some minor annoyances (some of the puzzles are created purely for gameplay, and don't make much sense in the story), and it's HARD (probably the hardest scenario, out of the box, I've seen), but it's really, really satisfying to beat. Overall, this thing is spectacular. Well worth the download. Comes with a new music file, not included in the above link. For full download options (including a single file including all pieces, as well as the music file separately, and a segmented chunks option), visit RED's Big House site.

Bighouse.bungie.org