Polymorphine is a liquid substance that transform the creatures it touches into harmless sheep.
There are 3 possible variants of sheep that a creature can turn into:
A sheep (of any type) has almost no health and will in practically every instance die in a single hit from anything. Additionally, the only method to be temporarily immune to the effects of polymorphine is to eat the dead body of a Master of Polymorphing, which means that this substance can be considered THE most dangerous & lethal material in the whole game.
However there are methods to be safer from polymorphine. Upon polymorphing or death, you drop all eggs in your inventory, including hollow eggs. You can put helpful spells inside these eggs that will be cast upon polymorphing, such as Circle of Transmogrification that will transform any melee enemies into harmless sheep. As for ranged enemies you can put in either Circle of Shielding that will bounce projectiles away, or Projectile Transmutation Field that will turn any nearby projectiles into sheep or ducks. This method is not perfect, but offers at least some protection when polymorphed.
Enemies and players can be polymorphed but bosses are immune.
Before Patch #8, polymorphing into a creature enabled you to have infinite flying due to the new shape not having a levitation limit. This has since been changed.
Can be mixed with Toxic Sludge to create Chaotic Polymorphine.