A fast-moving cloud of superheated volcanic gasses, ash, and rock that hug the ground and even the surface of the sea as they rush at speeds of up to 450 mph, destroying anything in their paths.
Pyroclastic flows are usually the result of centuries of
pent-up stress within
the volcano; thick, sticky magma plugs the vent of the volcano and thus builds greater and greater pressure until it explodes.
Usually the flow is denser than the surrounding air and instead of flowing vertically, they rush along the ground, sweeping over hills and
ridges like the said hills and ridges don't even exist.
Pyroclastic flows can reach temperatures as high as 1830 degrees F, and the average temperature is about half that; hot enough to
boil the blood in one's body and effectively flash-cook you alive. Some victims are charred while others get fatal steam burns.
In short, a far more forceful and dangerous force than lava and more difficult to escape, due to their speed and intense heat.
Example:
Pompeii and Herculaneum were destroyed in 79 CE when
Mount Vesuvius exploded, sending a pyroclastic flow barreling toward both towns, searing the lungs and burning the flesh of anybody who would not or could not flee.
May 8, 1902, about 30,000 people were killed in St. Pierre,
Martinique, when Mt. Pelee sent a pyroclastic flow over the city.
Mount St. Helens famously erupted with a pyroclastic flow turning the surrounding forest into a lunar landscape.
The fringes of the pyroclastic flow do not always cut a clear line between life and death; survivors at the fringes often endure hideous burns.