This mod is given at the penultimate mission of The Sacrifice, automatically ranked to level 5.
Umbral Fiber is essentially the Steel Fiber mod with Legendary rarity. It is considered the same mod as its Common counterpart, and thus both mods cannot be equipped together.
The set bonus for Umbral Fiber increases the armor given by the mod by 25% of mod's value when using 2 set pieces and 75% when using 3. At max rank with full set bonus, Umbral Fiber grants an armor increase of 192.5%, and increases Sentient damage resistance by 19.25%.
Resistance values in the above stat table are calculated based on set bonus values, displayed values are rounded to the first decimal place ingame.
This mod outperforms Steel Fiber from rank 8 when using 2 set pieces and from rank 6 when using 3.
Umbra mods have a unique polarity, whose matching polarity can currently only be found on Excalibur Umbra, Exalted Umbra Blade, and the Skiajati. It is not possible to (re-)add an polarity to a mod slot, so it's advised not to overwrite a pre-existing one.