Sentinel energy

Sentinel energy, also known as Wraithfire, is a type of fuel used to power Night Sentinel technology, harvested from Elemental Wraiths before their imprisonment by the Hell priests.

Lore
On Argent D'Nur, after the Argenta were able to defeat the Ancestrals and attain dominance over the world, they put resources toward researching the Elemental Wraiths. This led to gaining an understanding of magick and forming the Order of the Deag, as well as the Night Sentinels. Sentinel energy is apparently similar to Argent, both able to power huge machines, manifest weapons such as the Crucible blade, and offer medical benefits, healing disease and giving the Sentinels a boost of strength. Sentinel energy is found in-game in the form of Sentinel batteries which are used to unlock parts of the Fortress of Doom, as well as Sentinel Cores which are used to power sentinel technology during levels and advance further.

Once the Maykrs discovered Argent D'Nur and the power of Argent energy, they worked with the Hell priests to create Nekravol using Argenta slaves. There, Sentinel energy is combined with Hell essence to create Argent for both the Maykrs and the Night Sentinels, unaware of its true source.