Crystallized Fel dropped from Archimonde in Hellfire Citadel on Normal, Heroic and Mythic mode after complete Legendary Ring Quest. Only one drop per week among all difficulty levels.
Crystallized Fel will upgrade 3 lvl for your legendary ring, up to a maximum of 795 . But it may need 20 weeks... In current Patch, Crystallized Fel can be purchased from Dawn-Seeker Krisek in Tanaan Jungle and Zooti Fizzlefury in Talador for 1250 valor points, No weekly purchase cap. Means that you can totally get four Crystallized Fel in one week, three were purchased by valor points and one dropped from Archimonde. In another word, your legendary ring can be upgraded up to ilvl 795 in five weeks. The alteration is considerate a lot for those multi-spec classes. As long as you get enough valor points to rapidly upgrade legendary ring level, your alt's strength will also be upgraded a lot.
In early 110, ilvl 795 gears still worthy using. Before new expansion, getting your ilvl 795 legendary ring now!

Zooti Fizzlefury in Talador:

Dawn-Seeker Krisekin Tanaan Jungle: