The Little Mermaid
In The Little Mermaid Ursula tricks Ariel into giving Ursula her voice in exchange for legs in order to pursue Prince Eric. On the condition that she kiss Eric in three days of the transformation. If she does not kiss him she will become Ursula's forever, being transformed into a little shriveled polyp.
When Ariel gets too close to kissing Eric, Ursula turns into the beautiful Vanessa. Using Ariel’s voice to lure him to her, she cast a spell of bewitchment and entrancement upon him. After casting the spell she convinces him to marry her, now killing any chance of Ariel kissing him in time.
Ariel friends foil the wedding, but not in time to keep Ariel from becoming a polyp. Ariel’s fins and voice returns and Ursula’s true form is reviled. Ursula then takes her to her father, King Triton, and shows him his withering daughter. She then strikes a deal with him. She will release Ariel if Triton takes her place.
Triton agrees and Ariel is restored while her father starts withering instead. While Triton is too weak to rule Ursula takes the crown and the Triton and declares her self Queen of Atlantica.
However, soon after gaining power she is defeated by Price Eric. All of her evil doings are undone and King Triton is king once again.