The black fabric of the robe was inscribed with interwoven mithral threads in the shape of runes and sigils. These inscriptions allowed the wearer of the robes to cast the spells associated with their symbols. A robe could contain 10 such sigils, which would be consumed upon casting. Each time this was done a new rune or sigil could be woven in the previous symbol's place.
The sigils on these robes could be inscribed by skilled weavers who were capable of scribing scrolls.