Giant leeches are huge blood sucking predatory worms typically living in freshwater lakes. They are closely related to the oligochaetes appearing in Rowley Jefferson's Awesome Friendly Adventure.
Roland the Kind's group went through a tunnel in the Mines of Murlak with a shallow lake. The group crossed the lake only being covered in giant leeches. Roland remembers a time where he got one on an inappropriate spot during summer camp causing his mom to pick him up days early. Luckily the One-Eyed Wizard manages to solve the problem by zapping them however this causes the dwarves to be alerted of intruders.