summary what we know so far:
- the set drops as a chest called "Kor'kron Dark Shaman Treasure" from the boss as personal loot (much like Titan Runestones/Secrets of the Empire)
- only drops in flex, normal & heroic mode
- does not drop from any trash
- you only get one chance for this item to drop per week regardless of difficulty (1st time you kill Dark Shamans you get a chance, second time the same week no chance anymore)
- it is a very rare item (for me, very rare = ~5-7%)
- it looks amazing!
Basically the cache uses the same code like Titan Runestones/Secrets of the Empire. Only certain players are eligible to get a single chance per week.