Dennis Markowski was the former manager of Lavandería Brillante industrial laundromat, owned by Gustavo Fring, which served as a cover for the location of a meth superlab. He was taken into custody by the police and later killed by fellow inmates while in prison.
History
Season 4
When Steven Gomez and another DEA agent come to investigate the laundromat, Dennis initially refuses to let them snoop around. However, when Gomez threatens to get a warrant and bring in more DEA agents, Dennis nervously agrees. ("End Times")
Season 5
At the beginning of the fifth season, it is revealed that Dennis has been arrested and incarcerated for his connection to the Fring organization. Mike Ehrmantraut, posing as a paralegal, visits Dennis in prison to discuss his hazard pay. Dennis voices his concerns but promises Mike that he will keep his mouth shut and serve his probable eight years in prison. Mike in turn promises to recoup Dennis' lost funds from Gus' Drug Empire. ("Hazard Pay")
Dennis being burned to death in his cell
When his attorney comes under investigation and Mike is killed, however, Dennis prepares to tell the DEA what he knows in exchange for legal immunity. Hank Schrader vehemently denies this request, believing instead that he can coerce one of the other eight incarcerated henchmen without having to make such a generous deal. Before anyone can spill the beans, though, all of the henchmen are silenced in a carefully coordinated series of prison murders - Dennis is burned alive when other inmates douse his cell with a flammable liquid and ignite it. ("Gliding Over All")