well in kingdom you kill monsters and use their remains to brew something tasty
not sure if you league or empire
but as an example if you lerned Herbal Tea (its a rubee) you can collect water from the river as a league or blood from the kettle as a imperial to brew it
later on you can brew other potions too
if you wonder what you have to kill here a little list:
as a empire:
Herbal Tea (Healpotion) -> Blood from Kettle
Lethargie Potion (Long time cc) -> Crows
Refreshing Potion (Speedpotion) -> Frogs
Potion of Punishment (useless damage potion) -> grey wolves
Aqua Morte (-50% healing on enemy) -> league pets in their camp (kittens,squirrels,bears etc.)
as a league(sry its some years ago so i dont remember all):
Herbal Tea (Healpotion) -> Water from River
Lethargie Potion (Long time cc) -> mushrooms (not sure)
Refreshing Potion (Speedpotion) -> pluck berrys from bushes
Potion of Punishment (useless damage potion) -> ? probably black wolves
Aqua Morte (-50% healing on enemy) -> goblins in the empire camp
i hope this helps