Third party scenarios are not supported by Bungie, and require the purchase of the appropriate Marathon product in order to use them. Details on obtaining these scenarios are contained on their specific pages.
If there is a scenario that you would like to see featured here, please email firstname.lastname@example.org, and let me know the details of the scenario.
Created by the EVIL group, this 17 level solo scenario takes place after the events in "Seige of Nor'Korh", which itself is sometime after the events of Marathon Infinity. You and the last of the original Marathon BOB discover more than just the Pfhor on your way back to the Sol system - and maybe even deadilier. If you need help with this scenario, the Evil Spoiler Guide will help you along.
Marathon EVIL can be obtained at http://bighouse.bungie.org/evil.
The Tempus Irae scenario, created by the Nardo Group, has 36 solo levels, and 20 net levels. The game continues to use the same weapons and monsters as in the Marathon games, the setting has been moved back several centuries, and features amazing artwork and shapes from the Reinessence period as you try to keep time from collapsing in the hands of the Pfhor. The spoiler guide for Tempus Irae will help you through these levels.
Tempus Irae can be gotten at http://www.nardo.org/.
Origin of Species is a compilation of all the levels Tony Smith ever released publicly, plus several continuity levels added by the OoS team. These were some of the most enjoyable Marathon 2 levels ever released, and they've been updated to play under Infinity (with some minor bugs worked out of them), and some extra story written up to keep the flow going. The spoiler guide for Origin of Species will walk you through the hardest parts. (There is also an inventory of weapons and ammo, in case you're not sure if you found everything.)
Origin of Species can be gotten at The Archives at Bungie.org.
Marathon Rubicon is one of the most ambitious Marathon scenarios ever released - and it can also be one of the most confusing. Luckily, the creators have put together an easy-to-navigate, pretty-to-look-at spoiler guide which should get you through the worst of it. They'd be really happy if you didn't use it, though...
Marathon Rubicon can be gotten at http://www.marathonrubicon.com/.
Trojan was the last really big Marathon 1 scenario released - and one of the very few scenarios in ANY Marathon flavor that can legitimately claim the 'TC' moniker. (It's certainly the first.) It's currently available for M2/Win95... but not (legitimately) for either M2/Mac or Infinity. If you're having trouble with it (in either released flavor), this spoiler guide should help.
Trojan can be gotten at http://bighouse.bungie.org/trojan/ (for the Mac Marathon 1 version) or http://bighouse.bungie.org/tm2/ (for the M2/Win95 version).