Poison water is a spell solely usable while the Mephitic spellbook is open. Level 80 Magic is required to cast this spell. To cast this spell you must have at least one vial of water in your inventory. When it has been cast on a vial of water, the option screen will appear at the bottom of your page (like when cooking) it will give you the choice to poison 1, 5, 10 or X vials of water, turning them into either weapon poison, weapon poison + or weapon poison ++. The chances of it become weapon poison ++ is fairly slim (1/8), most commonly it will either be weapon poison or weapon poison +.