Medusa

Medusa is a Monster that appears in The Lightning Thief. Posing as a human, Medusa lures people into her 'mannequin' shop and then turn them into statues themselves by looking at them. At one point, Percy, Annabeth, and Grover are lured into the shop during their Quest. Medusa feins kindness, and then attempts to turn them into statues. After a long struggle, Percy defeats Medusa. Her severed head is later used by Percy for several purposes.

Also See
Medusa's Head