Circe, in Greek mythology, is a sorceress who lives on an island called Aeaea, near Italy. Circe was able to turn people into beasts (FW). Harry collected her Chocolate Frog Card during his first trip on the Hogwarts Express (PS6).
The original pronunciation is KEER-kee, but the name has been Anglicized to SEER-see.
In Homer's Odyssey, Circe fed magical herbs to the ship's crew of the hero Odysseus, turning the men into wild pigs. She tried to seduce Odysseus himself, but he first insisted that she turn his crew back into men.
From The Aeneid by Virgil:
Now near the shelves of Circe's shores they run,
(Circe the rich, the daughter of the Sun,)
A dang'rous coast: the goddess wastes her days
In joyous songs; the rocks resound her lays:
In spinning, or the loom, she spends the night,
And cedar brands supply her father's light.
From hence were heard, rebellowing to the main,
The roars of lions that refuse the chain,
The grunts of bristled boars, and groans of bears,
And herds of howling wolves that stun the sailors' ears.
These from their caverns, at the close of night,
Fill the sad isle with horror and affright.
Darkling they mourn their fate, whom Circe's pow'r,
(That watch'd the moon and planetary hour,)
With words and wicked herbs from humankind
Had alter'd, and in brutal shapes confin'd.
According to the Famous Wizard card, Circe is an "[e]xpert at turning lost sailors into pigs," an obvious reference to the story from the Odyssey. Odysseus escapes this fate by the use of a protective drug called "moly" which was given to him by Hermes.