The deific essence of the Forgotten Gods bleeds from their lasting wounds and seeps into the earth where they're entombed, running like veins of stone up to the surface. The crystal born from those veins is essentia, and all magic in the Mortal Realm comes from its power.
Raw essentia crystals can provide enormous power. Folk install raw or polished crystals in their armor or weapons; some who follow the "forgotten ways" use raw essentia in spiritual practice; others abstain entirely, believing it a profanity to the New Gods they worship.
Around Godfell Mountain the crystals are charged by Kudamat siphoning his power back through the veins — which Preacher Rayos preaches has made them more dangerous than they were.
Source · Drylands History