Umbral Fiber

Umbral Fiber is the Umbra variant of Steel Fiber and a legendary Warframe mod that increases armor, +100% at max rank, and adds Tau resistance. It belongs to the Umbral set with Umbral Intensify and Umbral Vitality, whose set bonus grows with each additional mod equipped.

You receive one at the penultimate mission of The Sacrifice quest, automatically ranked to level 5, and further unranked copies are sold by Cephalon Simaris for 100,000 standing. It is untradeable and cannot be transmuted. Equip all three Umbral mods together to get the strongest set effect.

Umbral Fiber was added with Update 23.0 (June 2018).