Umbrals are a type of infernal that occupies a distinct niche in the Outer Abyss, benefitting from being able to slip into Caen more easily than others. They possess a special connection to shadows that lets them exploit darkness to serve as gateways. Guardians are often contracted to bind into the shadows of important infernal assets, lurking unseen to await the time they are needed, at which point they spring into action in defense of their charge.
Umbral Guardians are incredibly potent defensive pieces that can appear near friendly models that are being attacked at range by your foe and absorb the shot. Umbral Guardians are at their most dangerous when being attacked, thanks to their Defensive Strike, which can make advancing up to them very perilous.