While Sean Cassidy was working on a deep undercover mission for INTERPOL, his wife Maeve gave birth to their daughter, Theresa. Not much later, Maeve died in an IRA bombing. With no means to contact Sean, his cousin Tom took care of Theresa. When Sean returned to learn of his wife’s death, he was devastated. Before Tom could even tell him of the existence of his daughter, Sean lashed out at Tom with his sonic scream for not having taken better care of Maeve. While Sean flew away in anger, Tom fell into a chasm, breaking his leg as a result of the attack, which left him with a limp. Angrily, Tom swore to make Sean pay and vowed to never tell him about his daughter, raising her himself instead.
Tom (now known as Black Tom) would raise Theresa as his apprentice in his criminal activities. Eventually, Theresa accompanied Black Tom and his then-partner, the Juggernaut, to San Francisco to assist him in his crimes. Here, employing the name Siryn, she battled the original Spider-Woman and several of the X-Men. This conflict ended in Black Tom's temporary capture. While in custody, Black Tom exonerated Theresa of responsibility for her crimes and wrote a letter to Sean explaining who she was. The X-Men brought Theresa back to their headquarters, where she was joyfully reunited with her father.