|Umbral energy is the darkness-based polarity present in all six elements (see Chapter 4 of Essences & Permutations - A Treatise of the Six Elements). The opposite of Umbral energy is Astral energy.
Umbral energy can exist in all 6 elements, as demonstrated by the even-numbered months of the Eorzean Calendar. Likewise, The Twelve associated with Umbral energy are Menphina (Ice), Nymeia (Water), Oschon (Wind), Rhalgr (Lightning), Nald'thal (Fire), and Althyk (Earth). It is most heavily utilized by Disciples of Magic, such as the Thaumaturge.
There is currently no proven way to increase a character's Umbral affinity. However there may be equipment or buffs available which enhance this attribute, and there are spells which affect the ability of enemies to resist Umbral-related effects.