Helstorm Rocket Battery

The Helstorm Rocket Battery was is an experimental artillery piece used by the Empire of Man as a means to shower the enemy with a hail of highly volatile rocket ammuntions. The Helstorm Rockets was first inspired by a particularly impressive display of fireworks in the city of Altdorf. Early prototypes blew apart an entire floor of the Imperial Engineers School, but the soot-blackened Engineers persevered and succeeded in creating a deadly, if unpredictable, weaponry. The shrieking rockets fired by this bizarre machines are wildly inaccurate weapons that have little chance of hitting anything other than the ground. However, when these rockets do land on its intended target, the results are devastating; entire regiments are blown apart by a series of earth-shattering eruptions by the explosive impact of these rockets. After Count Boris Todbringer was almost by an errant barrage, they were dubbed "Helstorm" rockets after the colorful language used to reprimand the unfortunate crewmen.

Source

 * Warhammer Armies: The Empire (8th Edition) -- p. 49