Quasit are soul worms formed by Demon Lords for service as familiars to chaotic evil clerics and magic users. At will, quasits are able to polymorph themselves into any 2 of the following creatures: frog, bat, giant centipede, or wolf.

Despite their rather low intelligence, quasits are quite crafty and cunning creatures. They are also used as tools by demon lords, who might communicate through them. Quasits are generally tasked with helping their masters to spread evil and chaos. In particular, they are often used to pervert lawful evil souls to chaos, which earns them rewards on their home plane of the Abyss. Ultimately, when its master dies, the quasit is charged with bringing the soul back to the Abyss. If the quasit has served well, it may be rewarded by being changed into a class A or class B demon. If not, they will be punished through becoming a kullule, soul worm, or even a quasit again.