How do you get a lot of mana potion in aq

you go to the paladin armoury in yulgars inn and buy the spell "light of mana" after that go to a place where you could battle undead such as granemor which is on the left side of travel map and is under darkrovia when your in granemor click on the grave on the right side and battle monsters use the spell and you will get 1 mana potion sometimes the spell doesnt give you a mana potion and this only works on undead