Ever since the tieflings of Cael'doresh broke their pacts with devil kind, destroying or sealing away the portals to those infernal realms, children have been warned of the umbral. Practice whatever discipline you prescribe to, meditate upon this fact that if you meet an umbral in the the shadows, you are dead. Shadows seem to exist for them as they as air does for us to breathe and even the brightest light casts a shadow.
What little can be surmised of these creatures is that they live and move through shadow. In appearance, they are shadows moving on their own, twisting in the corners of your vision until they are ready to strike. An uncanny ability to adapt to attacks separates them from their devil comrades, more like demons in that respect. Umbral existed alongside the devils of old Cael'doresh, before the tieflings broke their pacts. Little is known of this time as many records of that time were destroyed.
If ever the umbral return it will be a grim time indeed, for can the devil be that far behind? There may be dark times ahead, but there will be heroes at hand who may prevail!