Also known as Helish Crusade. Helmarches are attempted incursions by armies or adventurers from Materia to attack, purge, destroy or occupy a plane of Hel. Many times this has involved taking a side in the Bloodwar by allying with Devils against Demons or vice versa.
Currently, the plane of Crimson is a plane of Hel. Some Crusaders have suggested doing a Helmarch to help destroy the Demonic Brigade once and for all.