Which witch turned Ariel, a mermaid from American animated film "The Little Mermaid", into a human?

Ursula is a fictional character who appears in Walt Disney Pictures' 28th animated feature film "The Little Mermaid" (1989). Voiced by American actress Pat Carroll, Ursula is a villainous sea witch who offers a mermaid princess named Ariel in exchange to trade her voice for a pair of human legs, at first appearing to be providing the character with an opportunity to become human by temporarily transforming her into one so that she may earn the love of Prince Eric within three days. However, Ursula is, in fact, determined to sabotage Ariel's chances so that she can ultimately replace King Triton as ruler over Atlantica.