Harry Potter and the Goblet of Fire
The trio secretly meet Sirius Black at Hogsmeade, who tells them that Barty Crouch was fanatical about catching dark wizards, often stooping to the same level to punish them. This culminated in Crouch sending his own son to Azkaban. His son later died there. Sirius suspects that someone is attempting to attack Harry through the Tournament and make it look like an accident. He suspects Karkaroff, who is a former Death Eater who walked free.