Liann Crosby is the main villainess from "Venom," episode 9.07 of Law & Order.
She was played by Laila Robins, who appeared earlier in the series as Diana Hawthorne.
Liann Crosby was introduced as a woman who was shot along with her companion, Stuart Wattman, with Liann surviving while Wattman was killed. In actuality, Liann's backstory revealed that she was born as Melissa Beck, and at some point, she gave birth to a son, Dennis Beck, and raised him on her own. Liann engaged in criminal behavior during Dennis' childhood, and even forced Dennis to become an accomplice, doing so as early as the age of six. She threatened to abandon Dennis if he didn't comply, and a twisted way of keeping Dennis in line, the villainess began an incestuous relationship with Dennis. The assaults began during Dennis' teenage years, and even continued in the episode's events.
Liann habitually married rich men for their money, with one of them being Henry Crosby, who Liann ordered Dennis to kill so she could inherit $18 million from him. Dennis does so by drowning Henry, and afterwards, Liann enlisted her son to marry wealthy Joyce Pollock, as she was their next target. However, Dennis developed true affection for Joyce and disobeyed Liann's order to kill her, leading to the evil Liann making an attempt herself by sabotaging her gas heater. Joyce survived Liann's attempt, and once Dennis found out, he went after Liann and held her at gunpoint. Dennis' shot ended up striking and killing Wattman, though he later shot Liann (non-fatally).
Liann was encountered by detectives Rey Curtis and Lennie Briscoe, though she constantly evaded their questions and later headed to Saratoga Springs. The investigation revealed Dennis' role in the shooting and uncovered his connection to Liann, though it was originally believed that they were husband and wife. After Dennis' charges were dropped, Liann appeared in the courtroom and outed Dennis as the shooter, doing so (as later revealed) to further gain control of her son and as payback for his disobedience (in her mind).
The villainess continued throwing Dennis under the bus, but their true familial ties and criminal history together ended up revealed, as well as Dennis revealing that Liann had been forcing him to commit crimes with her and that she molested him for years. Following Dennis' testimony, Liann took the stand and responded to Jack McCoy's questions about the murders and cons with denial, but surprisingly admitted to the incest and blaming herself for how Dennis turned out, while also revealing that she called Dennis and told him to meet her at Saratoga Springs. McCoy later met with Dennis and Joyce and offered the former a deal to testify against Liann, after it was revealed that Joyce devised her own plan to kill Liann. To the surprise of everyone at the meeting, including (and especially) Joyce, Dennis turned down the deal, and as a result, Liann avoided all charges.