Morrigan's History
Theresa Maeve Rourke Cassidy is the mutant daughter of Sean, the mutant who was to become known as the Banshee, and Maeve Rourke Cassidy. Soon after his marriage, Sean, an Interpol agent, was sent on an undercover mission for many months, not knowing that Maeve was pregnant when he left. Shortly after Theresa's birth, Maeve, an innocent bystander, was killed in an IRA (Irish Republican Army) bombing along with a number of other people. Sean's cousin, Tom Cassidy, got custody of the baby and raised her as his own, hiding the truth from Sean as well as Theresa for decades.
When Sean returned and learned of Maeve's death, he blamed Tom for not keeping the woman they both loved safe, becoming so filled with anguish when that he left his ancestral home of Cassidy Keep and threw himself into a series of highly dangerous missions, eventually leading to his resignation from Interpol and his later involvement with the mutant villain team Factor Three.
Siryn
Theresa's sonic powers emerged at puberty and, a few years later, Black Tom began coercing her aid in committing crimes, whom she called "uncle" out of respect. At one point during high school, Theresa developed a drinking problem. However, Theresa's heart was never in a life of crime and she pursued it because the man who raised her, Black Tom, ordered her to do so. Eventually, Theresa accompanied Black Tom and his partner the Juggernaut to San Francisco.
There, employing the name Siryn, Theresa 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 gentlemanly exonerated Theresa of responsibility for his crimes and wrote to Sean explaining who she was.[13] Theresa and Sean were joyfully united at Professor Charles Xavier's mansion. Theresa went to live with her father, who had since retired back at Cassidy Keep and, due to his new girlfriend Dr. Moira MacTaggart, divided their time between Cassidy Keep and Muir Island.
Fallen Angels
Some time afterward, Theresa lived for a while at the Muir Island Mutant Research Center, owned by Dr. MacTaggert. There, Theresa met and developed a romantic relationship with Jamie Madrox, the Multiple Man, unaware she was actually gettin involved with a dupe. After being informed that the New Mutant Sunspot had run away, Theresa and Madrox helped to track down the young mutant. Traveling to New York, Theresa and Jamie found Sunspot and Warlock, attempting to rob a church for food, and Gomi propositioned Bobby Da Costa to join the Fallen Angels. Vanisher appeared to further persuade the children and, while still considering, Chance and Ariel appeared and challenged Sunspot to take a chance. Then Bobby, Warlock, and the duplicate Madroxes left with the group.
After tracking the youths, Theresa and the original Madrox met Boom-Boom and were teleported to the Valley of the Flame, on Dinosaur World, where they were joined by Moon Boy and Devil Dinosaur. After Devil Dinosaur, accidentally, crushed Don the Lobster, Gomi's words of not leaving friends after an accident or tragedy struck a cord and Bobby began to second-guess his decision. After all of the mutant's powers, temporarily, went out-of-whack due to Chance, Ariel suggested that they all leave and teleported them to the Coconut Grove, her home planet. The inhabitants of the Coconut Grove and the leader, Unipar, captured the Fallen Angels, intending to use them as experiments to continue their evolution, but the Angels worked together to defeat them and return home. Sunspot and Warlock decided to return to the New Mutants, but Siryn and Madrox chose to remain with the Angels instead of going home to Muir Island.
Muir Island
Siryn eventually returned to Muir Island, only to get caught up in the Shadow King's telepathic influence. When the X-Men came to Muir to investigate, Siryn ended up fighting her own father Banshee, and took advantage of his feelings to beat him for the Shadow King. She was freed from Farouk's influence with the other residents after the Shadow King was defeated on the physical and astral planes.
X-Force
Later, Theresa learned of an attempt by Black Tom and the Juggernaut to bomb the World Trade Center in New York, and she joined the formative group of mutants known as X-Force in defeating them. She remained alongside the team for a long tenure, even becoming Deputy Leader after Cannonball's departure. On a solo adventure, Theresa encountered the mercenary Deadpool when both were tracking down activities of Black Tom. The two developed romantic feelings toward one another, but parted company.
During her time with X-Force, Theresa's teammate Warpath developed a crush on her, and later he helped Theresa recover from a relapse into alcoholism.
During a clash between X-Force and the villainous New Hellions, Theresa's vocal cords were severely damaged, causing her to lose her voice as well as her powers. Taking a leave of absence from the team, Theresa adjusted slowly to life without her powers, visiting with an aunt. Soon, however, Theresa walked into an antique store where the owner put a necklace on her. Theresa immediately began to talk again and flew off, the obvious victim of possession, similar to the type performed by the deceased villain Malice.
Later, Theresa encountered Deadpool who managed to get her to an organization that was able to heal her using the mutant healing power of their captive, the X-Men's Wolverine. Wolverine was upset at Deadpool, even after Theresa broke out of her healing tank, but Terry stood by Wade and flew off with him. What became of her possessor is unknown.
X-Corps & X-Corporation
Banshee founded a mutant team, the X-Corps, as a paramilitary force that could police the mutant community. The team recruited several known villains and soon they embarked on a rampage through Paris until the X-Men were able to defeat them. Even so, Banshee was left wounded and the X-Corps was disbanded. Feeling the need to help atone for her father's debacle, Theresa offered her services to the X-Corporation, a clandestine extension of the X-Men operating as mutant champions on a worldwide scale.
X-Factor Investigations
Following M-Day, Siryn became a member of X-Factor Investigations. As part of her duties, she investigated the murder of the woman who died in Jack Vaughn's penthouse under suspicious circumstances. Jack Vaughn (a movie star) claimed that the woman had been killed by her sister, Gloria Santiago, who was upset about the relationship between the two. Rival agency Singularity Investigations represented Jack Vaughn, with Damien Tryp, Jr. as the defense counsel, while Siryn and X-Factor Investigations tried to help Gloria. After Monet established a mental bond with Gloria to assess the truth, she managed to incriminate Vaughn, which made Tryp, Jr. so infuriated, he severely beat Siryn and left her for dead (having shot her in the neck with a dart that prevented her from using her powers). She was then captured by Doctor Leery, but, with the help of Rictor, she escaped, getting Leery killed in the process. After this, she entered a state of denial regarding her father's recent demise, correctly noting that many X-Men had been believed dead only to later return alive. Due to these recent traumas, Siryn was under considerable emotional strain.
After meeting Singularity Investigations' CEO, Tryp, Sr., Madrox started suspecting Singularity Investigations wasn't responsible for the decimation, as he'd previously thought, so he asked Siryn to find out more about it from the heroes who might have been involved (by using her modulated voice to make them more suggestible). She learned from Spider-Man, and later from Quiksilver, that the X-Men (who had denied any involvement) were, in fact, central to the ending of the House of M and the subsequent Decimation. After a confrontation, X-Factor took a public stance in defiance of the Superhuman Registration Act and the X-Men decided not to interfere with Quicksilver as long as he stayed in Mutant Town.
When X-Factor decided to break into Singularity Investigations, Siryn and Rictor were in charge of clearing the surveillance room, while the others retrieved files on the new virus being developed by the organization. After Tryp, Junior and Tryp, Senior showed up, Siryn was once again shot in the neck with a power-dampening dart, but this time she was equipped a kevlar collar that allowed her to strike back against her attackers. Before the confrontation could escalate, however, Jamie urged the team to retreat and deal with other pressing subjects. Unbeknownst to them, one of Madrox's dupes had been left behind and blew up the whole building, apparently killing Tryp, Junior and Tryp, Senior.
One night, Madrox got drunk; Jamie Prime and a dupe he described as his libido incarnate slept with Siryn and Monet. Jamie was not sure which of them slept with the dupe. Regardless, Jamie kept it going with the both of them, afraid of revealing that he was double-timing them. After the confrontation with Singularity Investigations, the members of X-Factor met with Doc Samson, who encouraged Jamie to confess the truth to Monet and Siryn. They did not take it well.
Despite initially being mad at Monet, the two realized their quarrel was illogical. Thus, they decided to go to Paris on a private jet and clear their minds together. After doing some shopping, they came across some anti-mutant riots. Later, finding themselves facing local authorities, they ended up fleeing the country in a hurry, bringing with them an ex-mutant named Nicole, who'd supposedly lost her parents in the riots. This girl was later revealed to be an android and part of an intricate plan to get rid of mutants for good, but Layla Miller dealt with it.
Shortly after returning from her trip with Monet, Theresa discovered that she was pregnant with Madrox's child.[35][36] After Mutant Town was destroyed by Arcade, Theresa and Jamie talked about their baby and their future together. The team also discussed what to do next and, in the end, they agreed to move their headquarters to Detroit, where Theresa spent most of her pregnancy.
Shortly before giving birth, Terry proposed to Jamie, who vehemently accepted, claiming he thought she'd never ask. With contractions becoming more frequent and Terry feeling more pain, she lost control and broke all the windows of the hospital, but Jamie never left her side, even in surgery. After a complicated delivery, she gave birth to a boy and finally acknowledged the death of her father. The baby was then named Sean, in his honor. As Jamie held his son for the first time, tragedy struck and he absorbed Sean, leaving Theresa in a deep state of sorrow.
Banshee
After disbanding the Detroit branch of X-Factor Theresa went back to Ireland where she ran into Reverend John Maddox, one of Jamie's dupes. Reverend Maddox helped Theresa cope with depression and she had a one-night stand with Deadpool. After finally coming to terms with Banshee's death and reconciling her grief, Theresa retired the codename Siryn and took her deceased father's codename as her own and began calling herself Banshee in honor of his memory.
The Morrigan
As Polaris continued to mentally deteriorate, Terry asked Layla Miller to help her contact the Morrigan in an attempt to save Lorna's sanity. The Morrigan revealed that her name was actually a title that was passed down from one woman to another and that she was tired of being the Morrigan. She would help Terry by giving her the power that was needed to heal Lorna's mind. To do this, Terry had to kill the Morrigan and become the new Morrigan. Theresa accepted and killed the Morrigan using a "Banshee's scream" in the form of a sonic lance, by the Morrigan's request. With her new godly powers, Terry easily saved Lorna.
After Terry donned the title, she immediately left the team, only reappearing when Jamie Madrox prayed to her while he was stuck in his demonic form because of Mephisto and the events of the contest to kill Tier Sinclair. She was initially surprised at the context of her summoning because she didn't expect this to be when Jamie summons her. She then healed Jamie and defeated the policemen that attempted to arrest Layla. She filled Layla and Jamie in on their friends' lives, and later offered to rally the team herself but didn't because she learned that Layla and Jamie just wanted to raise their child in peace.
House of X
Theresa, seemingly free of Morrigan's, joined the mutant state of Krakoa and reunited with her recently resurrected father at the celebration of Krakoa's recognition as a sovereign nation. She joined the island band and, using her sonic scream, worked in concert with Dazzler to produce a lightshow for all the mutants commemorating their new-found liberty.
Later, her father was hurt by Amneth's forces on a mission with Unus the untouchable and Summoner to the other side of the External Gate. Wanting revenge on those who hurt him, she decided to accompany another team through the gate to bring back Unus, who had been captured. As they were betrayed by Summoner, she blasted the first Horsemen of Apocalypse with her sonic scream, allowing her teammates to escape.
All-New X-Factor
Afterwards, a new X-Factor was created due to the influx of missing mutants and unknown deaths The Five couldn't process for the resurrection protocols. She was one of the first mutants on their investigation list after she let out a sonic scream as she fell to her death on a Krakoan cliff.
Resurrected by the Five, she died again 5 days later in England. Upon another resurrection, X-Factor interrogated her about her mysterious deaths, but she avoided their questions and blatantly lied to them. When Theresa left the Boneyard, she was confronted by Lorna on why she wasn't being honest, prompting her to use her powers to bewitch Lorna into sabotaging the investigation on her. It was later revealed Theresa was still under the control of Morrigan.
Noticing she was being "tailed" by Daken, she lured him into the Alberta wilderness, in the middle of a snowstorm, to kill him without having his teammates alerted, unaware he had a healing factor. He remained a few days out in the cold, impaled with a broken crossbeam, but was eventually rescued by Northstar, who had been told of Siryn's last whereabouts by Polaris after Rachel had telepathically removed the Morrigan's influence from her mind. By then, Theresa was nowhere to be found.
