Marching Ghost is a mob found in Forgotten Viaduct.
They spawn at the start of the round in detachments of 56 (4 rows of 14), at the southwest and northeast corners of the map respectively. From there they march towards the middle of the map, meeting in the middle right before the timer elapses completely.
Although difficult, they can be killed after damaging them enough. However, unlike the standard variant, the Marching Ghosts summoned by the Lord of Despair cannot be killed.