Lorn V is a forlorn ice planet that is the stage for Winter Assault. It is an old Imperial world taken by Chaos Space Marines at some point and later plagued by Ork tribes as well, both killing each other in minor conflicts.
Lorn V is mostly noteworthy for hiding a disabled Titan war machine left behind by the Imperium.
Lorn V was an Imperial world conquered by Chaos and never retaken, apparently because of its negligible significance to the Imperium. However, a damaged Titan of unknown classification – named the Dominatus – that fell during the defense of the world was left there and buried in ice.
In Winter Assault, the Cadian 412th Regiment of the Imperial Guard under General Sturnn is sent to recover the Dominatus for use in more important conflicts. However, the Chaos forces on Lorn V – a World Eaters splinter group led by Lord Crull – discover this motive and decide to claim the Titan for themselves (the Orks also find out about the Titan, but don't really care). Meanwhile, an Eldar force from Craftworld Ulthwé sneaks in, intending to use the Titan to deflect a Necron threat foreseen by their Farseers.
The player's actions may result in four different outcomes, but in the canonical version the Eldar prevail, defeating the Necrons but causing the Titan to be destroyed in the process. Presumably nobody will be interested in Lorn V after this.