From what I undestand if the black liquid affects the dragonblood and they want to find an antidote so they discover that the white powder. Which later on found as a drug and poison. He is basically using the council as a test subject on how lethal it was and once he confirmed that its has a same pontent impact on humans he uses his knights to spread it out now across the kingdom by mixing it on “salt” that is why ita no use to escape. Since they cannot stop or determine which is the condiment salt or the drug salt.
In my perspective since he lost his regas he no longer cares on anyone so hes basically slowly poisoning everyone which aligning again on the prophecy of driving the kingdom into ruin

I am confused