Poisons add plus points to your weapon, thus inflicting more damage when
attacking an opponent.  Poisons are sold in vials at the Apothecary in the
Square.  When you poison your weapon, there is a chance that you will empty the
vial and therefore have no more of that poison type.  The number of plus points
added to your weapon and the cost of a vial of poison depends on your character
class.  The maximum plus points a poison type can impose on a weapon is equal
to that weapon's class number, otherwise the rest will just drip off.