Mysterious Powder

SPOILER FOR: NEW PARTY MEMBERS 7/9/19 Grail traded Leky to the tribesmen[insert link to tribe page] for some mysterious powder that, when inhaled, causes the victim to go unconscious.

Questions?
Where did it come from? Why was this the reward for Leky? Do the tribesmen have more? Does anyone else in Rivergate know about this powder?