Cam someone spoil me about the truth of the Black Potion?

In short its from The Duke Blood. The main reason why he cannot mass produce it

Spoil



I read a precedent spoil saying it was Truyde's blood that became black after eating the decomposed body of the last king (so corpses of dragons giving the power to control dragons). That explain why the wooden item shows an infinite dragon eating itself and why the hawk's herbs are used

Eww he ate the kings dead body.