Siobhan McDougal, later renamed Siobhan Smythe, was the daughter of a Gaelic patriarch who was eligible to become the next leader of the clan after her father died. Her uncle, unwilling to let a woman lead the clan, interrupted the clan's supernatural ritual to induct Siobhan as the new leader, which ended with Siobhan being dragged to the underworld. An entity called the Crone offered Siobhan the chance to return to life as the Silver Banshee so long as she retrieved an eldritch book for the Crone.
This led the Silver Banshee to Metropolis, where she first faced Superman. The Man of Steel came out triumphant after some struggle and help from the Martian Manhunter. Silver Banshee has returned several times to plague Superman, Supergirl, and others, and she has committed several atrocities over the years.
10 The Inciting Ritual & The Crone's Deal
Becoming the chieftain of her Gaelic village was Siobhan's birthright, and it only went wrong when her uncle sabotaged the supernatural ritual that would have made Siobhan the matriarch of her village. That said, practicing eldritch rituals one barely understands seems like a bad idea in most cases. Making a deal with the Crone only made things worse, and history has proven all of these to be bad decisions. All of this led Siobhan to become the Silver Banshee and the atrocities she has committed since then.
9 Initial Metropolis Rampage
To hold up her deal with the Crone, Silver Banshee needed a mystic tome to return to the Crone. When she arrived in Metropolis, Siobhan began killing everyone in her way despite their having nothing to do with the curse that befell her. Banshee became a murderer, and this, of course, attracted the attention of one Superman.
8 Trying To Kill Superman
Banshee tried to kill Superman too, and, thanks to the magical nature of her abilities, almost succeeded. The Man of Steel narrowly avoided this fate and learned that the nature of the Silver Banshee's powers meant she couldn't kill the same person twice--which usually wouldn't be a drawback.
However, Siobhan believed that Superman was dead even though he wasn't, and Clark recruited his Justice League teammate, the Martian Manhunter, to masquerade as Superman's ghost and stop the Silver Banshee's initial reign of terror in Metropolis.
7 Possessing Mattie
Later, Batman helped Superman banish the Banshee back to the underworld, but Supergirl later rescued Siobhan when that dimension spilled into our own. Silver Banshee proceeded to possess a friend of Supergirl's named Mattie, bending the hapless girl to her will.
6 Trying To Get Mattie To Kill
The Banshee also played upon an old wound of Mattie's. Her brother was killed by a man named Gerald McFee who was then under the control of Gorilla Grodd's telepathy. However, Mattie never truly forgave Gerald, and the Silver Banshee tried to make Mattie kill him. However, Supergirl was able to get through to Banshee and get her to leave Mattie before Gerad's life could be claimed.
5 Teaming Up With Manchester Black
When the villainous Manchester Black began his personal war with Superman, he recruited several of Kal-El's old enemies to his cause. Silver Banshee was among the last wave that Black sent after Superman, along with Mongul, Master Jailer, and Bizarro. Manchester Black even gave Silver Banshee and edge over Superman by telling her Big Blue's true Kryptonian name, Kal-El. In the end, Superman was still victorious.
4 Taking Lex Luthor's Bounty
When Lex Luthor became President of the United States, he convinced the world that Superman had tried to kill him. This made the Man of Steel a fugitive. He also put a billion-dollar bounty on Superman and Batman's heads, as the latter was trying to help clear Superman's name. Silver Banshee took the bounty and was among the army of supervillains that assailed the World's Finest heroes.
3 Joining Up With The Secret Society
She also joined up with Lex Luthor's Secret Society of Supervillains that began to rain havoc all over the globe. In reality, however, it was all a scheme of Alexander Luthor of Earth-3 that led to Infinite Crisis. Silver Banshee was among the villains Luthor used to betray and capture Black Adam, whom Alexander Luthor needed for his grand machine to create the perfect Earth.
2 Trying To Kill Supergirl
Silver Banshee later turned on Supergirl in Supergirl #34, despite Kara previously helping Siobhan. This was likely due to the madness that the Silver Banshee curse had created in Siobhan.
Supergirl was able to win this confrontation, but the two would battle again later.
1 Trying To Kill Inspector Henderson
A man named Inspector Henderson came across several artifacts connected to the Silver Banshee and her curse. Henderson believes these artifacts can cure Banshee, and one gets embedded in his hand during an explosion. These artifacts attract the Banshee to Henderson, and Supergirl is forced to stop Siobhan from killing the inspector. In the end, Henderson is able to use the artifacts to help clear Siobhan's head, though he couldn't completely lift the curse. However, Silver Banshee thanks Henderson for what he has done and says she owes him.
As you can see, Silver Banshee's record isn't as extensive or brutal as many of her contemporaries. With the New 52, Silver Banshee, now named Siobhan Smythe, actually becomes an ally of Supergirl, and Siobhan's curse instead comes from her father. Siobhan's father is the Black Banshee, and Silver Banshee helps Supergirl battle him. She also helps free Supergirl from the rage of the Red Lantern ring that possesses her.