The Siren, using reflective surfaces as portals, was able to board the 17th-century pirate ship. She identified the ill and the injured and took skin samples from them. This process left a black spot on their palms. She then enticed the sailors to touch her - a task made easy by her beauty, seductive singing and mental hold over selected members of the crew. Finally, this touch allowed her to transport them to an incredibly advanced medical bay on a spaceship where they received high-tech supervision.
The Siren was misguided although not evil. She was terrifying but angelic, becoming demonic if you crossed her, yet ultimately she saved Rory’s life and later allowed Amy to care for him. But only after she’d signed a consent form.