-Warflash has compiled and written a much better guide for this HERE which includes rates & aquisition, please refer to that instead!
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Item: Crystal of Unstable Darkness - https://bdolytics.com/en/EU/db/item/42293/
Using this item accepts the quest: "[Alchemy] The Truth of Darkness"
This quest gives you a Kzarka Summon Scroll which summons the World Boss Kzarka on all of the regions channels - normal rules apply in regards to arsha & Node Wars.
Once the scroll is used the quest is completed and you can claim your reward.
Rewards: Kzarka Weapon Box + Vanquisher of Darkness-Title(Colorued?) + 10x Kzarka Latent Aura + 1x Splendid Alchemy Stone of choice (Green/Yellow/Purple)
Completing the quest unlocks a repeatable quest with identical rewards called "[Alchemy] Incomplete Evil God" which requires another "Crystal of Unstable Darkness".
The Crystal of Unstable Darkness is persumed to be acquired by combining four out of the five available Crystals all of which are extremely rare and obtained through various methods of alchemy/processing.
So far only 4/5 of the "fickle-five" have a known method of acquisition:
1. Crystal of Depravity - https://bdolytics.com/en/EU/db/item/42294/ - Obtained from Imperial Delivery of (old) alchemy boxes
2. Crystal of Corruption - https://bdolytics.com/en/EU/db/item/42295/ - A mastery-related byproduct from one of the reward-pools with a 0.0001% to be picked.
3. Crystal of Wickedness - https://bdolytics.com/en/EU/db/item/42296/ - Obtained by grinding Alchemy Stones (Green/Purple/Yellow) & Crafted Spirit Stones.
4. Crystal of Repugnance - https://bdolytics.com/en/EU/db/item/42297/ - Obtained as a non-mastery related "byproduct" while crafting Spirit Stones/Alchemy Stones.
5. Crystal of Deterioration - https://bdolytics.com/en/EU/db/item/42298 - ??? Unknown ???