The Darden family have been through some devastating events on Chicago Fire. Their bad luck was foretold in the pilot episode, when Andy Darden (Chris Sorenson) died during a particularly hectic fire. His death wound up being crucial to the bond between Casey (Jesse Spencer) and Severide (Taylor Kinney), but it also left the Darden children without a father.
Andy's wife, Heather Darden (Chaon Cross) would stick around for another season, and go to Casey for advice on occassion, but she eventually decided to leave Chicago to start a new life with her two sons. The next time we heard about her, however, she was in trouble with the law...
Heather Darden was arrested for drunk driving in season 2
It may seem like a minor detail to focus on, but Heather's legal troubles set off the chain reaction that led to Casey leaving Chicago and moving Oregon to watch over the Darden boys. And given how that move is starting to impact other members of the Chicago Fire cast (i.e. Sylvie Brett), we'd say it was a pretty important storyline.
So what happened? How did Heather Darden break the law and how long is she going away for? Well, as shown in the season 2 episode "A Problem House", Heather developed a bad habit of drinking and driving. She crashed her car while under the influence of alcohol, and wound up causing the death of the woman in the passenger seat.
Heather was sentenced to fifteen months in jail for DUI manslaughter, though she gets a reduced sentence for good behavior. She decides to make the aforementioned move out of Chicago after her release. Sadly, the move did not bring about change. When Casey gets word of Heather in season 10, he discovers that she has continued to drink and drive, and is now serving prison time in Oregon.
Heather's prison sentence led to Casey leaving Chicago Fire
Casey originally planned to relocate the Darden boys to Chicago, but was urged to move to their home town so that they could finish school there and maintain some semblance of normality. We technically haven't seen Heather since season 2, and given that most of Casey's adventures in Oregon takes place offscreen, we doubt we will see her again.
That being said, we hope the character is able to turn things around and reunite/reconcile with her children. Casey has served as a father figure in the interim, but there's no substitute for the real thing. Besides, the Darden family deserves a break.