When the ground shakes like a cavalry charge but the accompanying noise is a bovine bellow rather than the shrieking of horses, you know that a Measurer is on the prowl.
These enormous undead, fully seven feet tall if they were ever to dismount from the jet black ox they ride, rampage across the countryside spearing any living person they meet. Each harpooned victim is pulled in & tested in the coffin the Measurer carries in its other hand. The slaughter goes on until one of the slain fits, at which point rider, mount & grisly prize all vanish in a gout of crimson flame & greasy smoke.
Where do they come from? Who makes the coffins? Do the coffins fit a certain person, or do the Measurers simply kill until one of the corpses has the right proportions? No one knows & all efforts to find out have ended very, very badly.