Siren

Siren is an Artifice and a major element in Xenoblade Chronicles 2 and Torna ~ The Golden Country. Both Aegises have their own Siren: Mythra uses a white Siren, while Malos uses a black one.

Story
During the Aegis War, Malos used his Siren to destroy Titans, while Mythra would often use her Siren's power without restraint - something which her companions often criticized her for. When the party confronts Malos at the Kingdom of Torna's capital, Auresco, he admits that he got bored of using Siren for destruction, and decided to instead destroy Torna by awakening its true form and sending his Gargoyles to unleash its energy all at once.

During the battle at the Soaring Rostrum, both Malos and Mythra called upon their Sirens to do battle. After a brief scuffle, Malos urges Mythra to use her full power; when she refuses, he promptly uses his Siren to destroy Auresco. Horrified, Mythra unleashes a portion of Pneuma's power to attack Malos relentlessly before they both enter their respective Sirens to continue fighting, despite Addam's objections. Eventually, Mythra manages to destroy Malos' Siren, sending them both plummeting into the Cloud Sea, but the aftershocks of their battle destroy Torna's core, causing it to sink into the Cloud Sea as well.

When Mythra reawakens at Olethro Ruins in an effort to protect Rex from Malos and Akhos, she uses Siren's power to kill both of their Blades, Sever and Obrona.

While the party is battling Patroka and Mikhail at the Ardanian Factory, Mythra uses Siren's power again to kill their respective Blades, Perdido, and Cressidus, enraging Patroka, and forcing the duo to retreat.

Later when the party is confronted by Jin at Genbu's Crown, Rex and Mythra attempt to use Siren's power, but Jin, using his true form (which has the power to control all elementary particles), is able to deflect its particle cannon. Jin overpowers the two, and Mythra reverts back to Pyra. As Jin is about kill Rex by removing the Core Crystal that is supporting his life, Pyra uses her willpower to control Siren and threatens to commit suicide with its targeting ray if he doesn't spare Rex's life, prompting Jin to back off. Pyra releases control of Siren and is taken by Jin.

At Gotrock Oracle Ruins, after Pyra and Mythra fuse into Pneuma and defeat Jin, Malos summons Ophion to attack the party; Siren intervenes and does battle with Ophion, resulting the defeat of Ophion as well as the destruction of a portion of Gotrock Oracle Ruins.

As the party climbs the World Tree, Amalthus takes control of Mythra using Haze's Core Crystal and attempts to force her to use Siren to destroy Torna, but she resists. Ultimately, Amalthus loses control of her when Rex and Pneuma (with Poppi's help) are able to use Siren's power to destroy the amplification towers on Indol.

In the year 20XX, several Echelon Siren models are engaged over the First Low Orbit Station against the Saviorite Rebels. The rebels manage to destroy a number of Sirens, prompting the director of the station to give the order to activate Artifice Aion through the Conduit; however, he is locked out by professor Klaus, who instead uses the Conduit in his experiment to create a new universe.

Several Siren models along with several Gargoyle models are seen attacking Alrest's nations as Malos prepares to take control of Aion to destroy Alrest. Ultimately, just as the Conduit disappears, Rex and Pneuma manage to use Siren's power one last time to defeat Aion. With the Conduit gone, every Artifice ceases to function and begins to fall to Alrest.