Liann Crosby is the main villainess from "Venom," episode 9.07 of Law & Order. She is a con artist and serial killer who marries and murders wealthy men for their money with the help of her son (and lover) Dennis.
She was played by Laila Robins, who appeared in an earlier episode as corrupt attorney Diana Hawthorne.
Liann Crosby is 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 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 her accomplice, doing so as early as the age of six. She threatened to abandon Dennis if he didn't comply, and began having sex with him when he reached puberty as a twisted way of keeping him in line.
Liann habitually marries (and kills) rich men for their money, with one of them being Henry Crosby, whom she orders Dennis to kill so she can inherit $18 million. Dennis does so by drowning Henry, and afterwards, Liann enlisted her son to marry their next wealthy target, Joyce Pollock. However, Dennis develops genuine feelings for Joyce and disobeys Liann's order to kill her, so Liann tries to kill Joyce herself herself by sabotaging her gas heater. Joyce survives Liann's attempt on her life, however, and once Dennis finds out what his mother did, he pulls a gun on her and opens fire. His attempt on his mother's life is unsuccessful, however; the bullet passes through her and hits Wattman, killing him.
Detectives Rey Curtis and Lennie Briscoe question Liann at the crime scene, but she dodges their questions their questions and later heads to Saratoga Springs. The investigation reveals Dennis' role in the shooting as well as his connection to Liann, though Briscoe and Curtis initially believe that they married. After the DA's office drops murder charges against Dennis, Liann appears in the courtroom and outs Dennis as the shooter, doing so (as later revealed) to further gain control of her son and as punishment for his disobedience.
Liann continued throwing Dennis under the bus, but their true familial ties and criminal history together are eventually revealed, as well as Dennis revealing that Liann had been forcing him to commit crimes with her and that she had sexually abused him for years. Following Dennis' testimony, Liann takes the stand and denies her role in the murder, but admits to the incest and blames herself for how Dennis turned out, while also revealing that she called Dennis and told him to meet her at Saratoga Springs. Assistant District Attorney Jack McCoy later meets with Dennis and Joyce and offers the former a deal to testify against Liann, after it is revealed that Joyce devised her own plan to kill Liann. To the surprise of everyone at the meeting, including (and especially) Joyce, Dennis turns down the deal, and as a result, Liann gets away with the murders.
- Liann is loosely based upon the late con artist and murderer Sante Kimes.