Urgall Depression

The Urgall Depression was a vast basin on the world of Isstvan V, the site of the massed loyalist landing and subsequent betrayal of 30k. It is attested primarily in the context of the Drop Site Massacre, the opening act of open warfare in the Horus Heresy, and is not otherwise described as a location of note beyond that conflict.

The depression’s scale was such that it could accommodate the simultaneous deployment of several Space Marine Legions alongside supporting Imperial Army and Mechanicum forces, including armour, artillery, and fortified command positions established by the ostensibly besieged Warmaster. Its terrain is described as broken and expansive, features that shaped the dispersal and subsequent encirclement of the loyalist landing force during the battle.

In the aftermath of the massacre, the Urgall Depression became emblematic within Imperial and Legiones Astartes memory as the site of the Heresy’s defining betrayal, its name thereafter closely associated with the destruction of loyalist strength at Isstvan V and the beginning of open galactic civil war.