Samantha Slayer

Samantha Slayer, whose real name is Samantha Rose, is the main antagonist of the movies Samantha: Haven's Worst Nightmare and The Return of Samantha.

She was a member of Salem's Inner Circle, the leader of the Rose Slayer Tribe and the twin sister of Summer Rose who possessed the power of the Summer Maiden.

Past
In her early years, Samantha assisted Haven Academy while her twin sister Summer went to Beacon Academy; both twin sisters has becomed leaders of their teams and eventually graduated, becoming Huntresses.

Shortly after Raven left Taiyang, Samantha used to visit Summer's family and help her raising Yang.

At some point, Samantha witnessed Summer's abilities through the Silver-Eyes, thing that lead her to desperately obtain that ability at any cost.

Shortly after Summer disappeared in her last mission, she, like Taiyang and Ruby, couldn't handle the situation and escaped, disappearing from everyone's life.

That aftermath would eventually lead her to slowly go to the path of evil, forming her own tribe near Haven and changing her last name to Slayer.

It is unknown how and when she obtained the power of the Summer Maiden but such acts with that power would call Salem's attention.

Arthur Watts was the one who offered her to work for Salem, she then accepts, becoming part of Salem's Inner Circle.

Samantha: Haven's Worst Nightmare
Samantha was sent by Salem in replacement of Cinder during her rehabilitation to await for Ruby to arrive in Mistral.

One year passed since then, she orders her Tribe to ambush Ruby, who was sharing a nice Hot Chocolate cup with Weiss and Yang. With the trio missing, Samantha orders Hope to look for the girls as she has a little "reunion" with Qrow.

After a talk between her and Qrow, who was assisted by Oscar, Jaune, Nora and Ren, begins a wild battle that ends on Samantha's advantage, the five of them falls defeated as Samantha turns around to see how her Tribe is being killed one-by-one by an "unexpected guest".

Effortlessly, Samantha defeats Blake by stabbing her with her arm and takes her apparently dead body to the snowy mountains where Ruby, Weiss and Yang were located and throws it near them, causing an enraged Yang to confront her for killing Blake with no regrets, Samantha mockingly asks who she is and she attacks her in response, causing a battle to ensue. She easily overpowers and toys with Yang, but starts getting serious upon her demand.

Weiss jumps to attack Samantha but she easily knocks out the ice queen with an strong kick; immediately, she reveals her true powers to her nieces by throwing off her mask, revealing herself to be the Summer Maiden, also explaining her plan to obtain the power of the four maidens and the Silver Eye Huntress as well to become the ultimate being.

The two sisters engages their aunt in another duel with no much advantage on their side, Yang activates her Semblance once again, balancing a bit the fight; after another battle, Samantha grabs Yang in the face and throw her at Ruby, who awakens the power of the Silver Eyes and decides to avenge her teammates.

Though Samantha at first shows an usual confidence in the fight, she loses it when a piece of her dust Omen disintegrates, which Ruby surmises is because her Aura cannot protect it. Ruby accuses Samantha of turning herself into a monster in the name of power, forgetting about her bloodline as well.

Though Ruby and Samantha initially prove evenly matched, Ruby manages to gain the upper hand, with Samantha figuring out she's not ready yet to hold her own powers against the Silver Eyes. Exhausted from the fight and with her Aura severely weakened, Samantha fires one last shot from a fire blast, for her bad luck, Ruby dodges it and the melting ice falls on the Summer Maiden, freezing her body as she's being pushed by the snow to a deep abyss, Ruby calmingly walk and watches her aunt fall into the abyss, believing to have killed her before break down emotionally.

RWBY All-Stars 2: Maiden's Heritage
She appears in her Summer Maiden form as a playable DLC character and a boss character of Samantha: Haven's Worst Nightmare DLC chapter in Story Mode.

Trivia

 * Since the RWBY Movieverse doesn't took place in the RWBY main timeline and it's Non-Canon, she can be considered a Fake Summer Maiden.