Armour

The warplate of Russ is a suit of unique artificer power armour whose origins can be traced back to the mysterious period of bitter warfare which shrouds much of the Legion's history in the decades after the Primarch took over the VI'h, and is accounted for now within the Legion's chronicles by obscure metaphor and strange and macabre allegories impenetrable to outsiders. Within its shell are strange exothermic field generators, otherwise unknown of in the Imperium's arsenal of technology, which bleed energy, specifically heat and kinetic potential from their surroundings when triggered, with the effect of creating an aura of murderous chill around the Primarch at will. It is this effect which gives the armour its name to the Fenrisians - 'Elavagar'- a wave of killing frost.

Horus Heresy Book 7: Inferno