The Great Modron March
The Great Modron March refers to two things relating to Dungeons & Dragons, and specifically the lore of Planescape as a whole.
Firstly, it refers to an event that occurs every 289 years, when a massive army of modrons pours out of Mechanus on a grand campaign across the Great Wheel, sweeping through the planes and levelling everything in its path that either is stupid enough to try and attack them or which just doesn't get out of the way fast enough. It travels through every gate town in the Outlands and all of the Outer Planes, in this exact itinerary:
- Automata to Fortitude
- Arcadia to Mount Celestia, through Abellio and towards the town of Heart's Faith in Lunia
- Excelsior to Tradegate
- Bytopia to Elysium, via Dothion through the location of the Golden Hills and through Amoria via the River Oceanus
- Ecstasy to Faunel
- The Beastlands to Arborea, via Krigala
- Sylvania to Glorium
- Ysgard to Limbo, via a branch of Yggdrasil
- Xaos to Bedlam
- Pandemonium to the Abyss, via Pandesmos and through the Plain of Infinite Portals
- Plague-Mort to Curst
- Carceri to Hades, exiting Othrys via Undermountain
- Hopeless to Torch
- Gehenna to the Nine Hells, through Khalas
- Ribcage to Rigus
- Acheron to Mechanus, via the Blue Cube in Avalas, through the Mines of Marsellin in Thuldanin, and finally back to Regulus.
Aside from marking every 17th Grand Cycle of Mechanus (the time it takes for the largest gear of Mechanus to turn once, which takes 17 Cycles of 17 Years), nobody knows why the Modrons do this or what they hope to achieve. They just know it's a bloody inconvenience.
The adventure module of the same name revolves around the party accidentally encountering the Great Modron March on various planes are it mysteriously is launched years before it normally should have occurred. This is later revealed to be a result of Orcus as Thanatos murdering Primus and hijacking his identity so he can force the modrons to look for the components of his iconic artifact-tier wand.