A small round cantine, that is strapped around its wearer with a leather strip. It has the ability to never run dry. It will be able to contain any type of liquid such as acid or lava. It will also never run out.
All spell casters gain their abilities through a form of demon possession. So in order to gain magical abilites, PCs must track down and trap demons. Of course there are some pretty powerful demons, if the demon is stronger than the PC, the PC could find themselves subjugated to the Demon's will.