Said to have been the result of an alchemist seeking a way to dispose of some of the more toxic byproducts of his research, the drain leech escaped the laboratory in which they were created, and have since inhabited the sewers of larger cities.

They are occasionally known to be aggressive, attacking and attaching themselves to warm-blooded prey. They use their lobefin "wings" to launch themselves out of pools of water at high speed, and to crawl back to their pond if their strike is unsuccessful.

The dirtier and more foul the sewer, the more likely it is that a leech is happily lurking in a nearby pile of refuse. They especially like to attack in the dark.