Unstable Powerful Weapon Crystal

Unstable Powerful Weapon Crystals are what remains of Powerful Weapon Crystals, which can no longer be crafted. Prior to Shadowlands, using this item "restored" it into a Weapon Crystal and 25 Sorcerous Air.

Source
The original version of this item, Powerful Weapon Crystal, was crafted by scribes with a skill level of 1. The recipe was learned from Technique: Powerful Weapon Crystal, which could be purchased from the Scribe's Quarters at Lunarfall or Frostwall, or from the inscription vendor in Stormshield or Warspear.