It is a physical mark (a faint, tiny anchor point) placed there by the warlock minions of Reek, and left on the wall, or catapulted over as part of an attack. The Rot Mark is an mark of corruption put on gear and left to be recovered and reused by the hospitalers and reclaimers on The Autumn Wall. It is a curse that causes living things it touches and is near to rot and degrade. Rot Mark is known to be the heart of the power for The Suffering Shard.
Pages on the web site lean towards i20™ text flavor, discrepancies will be noted in the iCore™ tab.