Those who hunt by night will tell you that the wildest, most vicious of all animals is the tiny shrew.
The shrew feeds only by the dark of night. He must eat his own body weight every few hours or stave to death. The ravenous shrew eats everything, bones, flesh, marrow… everything.
Intro from Attack of the Killer Shrews 1959
Draw the party together as debtors to the local lord, king, head of the thieves guild, or mercenary clan, who is forcing them to work together to pay off their debts.