In Greek folklore, the shape-shifting demon with one bronze leg who serves Hecate is Empusa.
Empusa, also spelled Empousa, is a female supernatural being in Greek mythology whose nature changes across ancient sources. She is described as a shape-shifter with one leg of copper or bronze and one leg sometimes represented as an animal limb. Ancient writers connect her with Hecate, the goddess associated with crossroads, magic, ghosts, and the underworld.
The earliest surviving literary treatment appears in Aristophanes’ comedy The Frogs, where Empusa frightens travelers with shifting appearances. Later traditions portray empusai as seductive phantoms that prey on young men, bringing them closer to the broader family of dangerous female beings such as lamiai and mormolykeia.
The bronze leg is the key identifying detail in this question. A Gorgon is recognized by snakes and a petrifying gaze; a Siren is associated with alluring song; and Lamia is a separate child-devouring figure whose stories later overlapped with other Greek monsters. Modern fantasy often blends these beings together, but the Hecate connection and metallic leg point specifically to Empusa.