Crystallized Fel

Crystallized Fel is used to upgrade the player's legendary ring: Sanctus, Sigil of the Unbroken, Thorasus, the Stone Heart of Draenor, Etheralus, the Eternal Reward, Maalus, the Blood Drinker or Nithramus, the All-Seer.

Source
This item drops from Archimonde in on Normal, Heroic and Mythic mode once every week after completing We Don't Need No Library Card. It can also be purchased from Dawn-Seeker Krisek in Tanaan Jungle and Zooti Fizzlefury in Talador for.

It does not drop from the LFR version of the encounter.