Comment by Deleted User on Regas - Chapter 88

—The black potion is actually Duke Truyde itself. His blood. It contains the potent power of the dragon after consuming the human with the dragon's blessing's (power) 'corpse', and he's blood became the poison itself. That's why when Truyde produced a large amount of black potion he'll need a certain period of time before he could produce another amount of black potion again (because he needs to regain back the blood he lost). That's been going on from his ancestors up till the current Duke Truyde's generation, they've been eating the corpse of all the previous kings that they killed. It was said that the only thing that could leash the dragon's power was their power itself.

The thing behind why the "rare" herb that could cure the crow even if they eat a 'spoiled meat' is being emphasized and the mystery about the 'addictive' meat (where Iray is so desperate to have) foreshadows 'cannibalism'. I don't want to spoil further so that y'all could atleast have the thrill to uncover the whole story, but if you really want to know you can search it up in the Novel Updates forum, everything was already discussed in there.