The Netherlight Crucible allows you to upgrade your Artifact to unlock a new way to customize your Relics.
The Netherlight Crucible is an ancient remnant of Argus that once served as a source of considerable magic. Though its potential power is thought by many to be a gift, empowering it would require great sacrifice...as well as a delicate balance of Light and Shadow.
During the Argus Campaign, Alleria Windrunner slew Nhal'athoth and absorbed the void energy of its heart. After accomplishing her task, Alleria sent an adventurer to give the Remnant of Nhal'athoth to Velen and to tell him of what they have learned about the void incursions on Mac'Aree. From the information that the adventurer gathered, Velen came to the conclusion that the Light and Shadow must coexist. Velen believed that the Essence of Shadow taken from Nhal'athoth's remnants will be exactly what they need to bring balance to the Netherlight Crucible and give them the edge they need against the Burning Legion.
Back at the Vindicaar, Turalyon believed it to be the right time to revive Xe'ra. After assembling Xe'ra's components and combining them with Light's Heart, Xe'ra was consequently restored. After Turalyon welcomed Xe'ra back, Xe'ra expressed its delight that they have found its Chosen One — Illidan Stormrage. Xe'ra wanted to restore Illidan to his original state, before his demonic transformation and when he had golden eyes that signified his great and prosperous future. Illidan however, was reluctant to be healed by Xe'ra's Light and become the Chosen One Xe'ra is trying to make him become. Rather, he is content with seeking the Legion's end in his own form, regardless of how demonic or grotesque it has become. Xe'ra is insistent, however, and tried to transform Illidan with its Light by force. When Xe'ra bound Illidan and attempted to impose its version of Illidan's destiny, Illidan resisted and destroyed Xe'ra with his Eye Beam. Turalyon was enraged by Xe'ra's destruction and struck Illidan with his sword. However, Illidan caught Turalyon's blow and rebuked him for his blind faith in Xe'ra's prophecy. Illidan stated to all that, "There can be no chosen one. Only we... can save ourselves."
Prophet Velen sensed that Xe'ra's death had distressed Turalyon and many others and moved to uplift their spirits. Velen told Turalyon that the Light did not die with the Prime Naaru and that it still shined within all of them. Furthermore, it was imperative that they press on as they still had a world to save. However, Velen would not let Xe'ra's passing go to waste. Velen gathered the essence of Xe'ra's Light and imbued the Netherlight Crucible with it.
With the Netherlight Crucible upgraded, the weapons of the Vindicaar's champions can thus be imbued by both the Light and the Shadow.