Etherium is an energy source in the form of glowing green rocks. Both Sagan-1 and Borealis contain mineable Etherium. It is an invaluable resource across the galaxy because of its rarity and broad range of uses.
Etherium can be used in three ways: Biologically, mechanically, and mentally.
The Fellwright use Etherium biologically, inserting it into the bodies of trained worms which they then control. Without a proper resistance to the substance, Etherium inside the body creates bloating and pustules, as exhibited by Spitterbugs and Mindbanes. It boosts physical abilities and in some cases can grant new abilities unique to the physiology of its user. Mindbanes were humans, with superboosts psyonic abilities thanks to the chunk of Etherium sticking out of their heads.
Most of the Iron Marines and the Peripheroids use Etherium mechanically, as an energy source. It is an incredibly energy-dense substance that can be combusted for energy, either used as a battery by itself, or laced in bullets and swords to enhance its destructive power. Etherium used this way burns fast, and is the reason why the Iron Marines have to constantly mine it.
Lastly, the Empyreans use Etherium mentally. Each Empyrean soldier possesses a purified sphere of Etherium in the form of a little glowing belt with which they have trained for thousands of hours. Through a mental link they can access the Etherium's energy and manifest it in different ways. Guardians, for example, manifest this energy as their green swords. Etherium used this way lasts for centuries, and its user suffers no ill effects. However, if separated from its power source, the user loses most of their powers, though some remain.