As a sea witch, Ursula has certain powers that made her a threat to King Triton:

  1. She can conjure a magic bubble that allows her to spy on people. She used this bubble to spy on Ariel.
  2. She can conjure magical contracts that, when signed, cannot be broken. She tricked Ariel into signing one when she agreed to turn Ariel into a human.
  3. She can transform merpeople into pathetic little polyps which she calls "poor unfortunate souls." She used this spell also to permanently wither Ariel into a "poor unfortunate soul" so that her father will take her place and give her his power.
  4. Ursula can also transform both herself and other merpeople into humans. She used this power to turn both Ariel and herself into humans.
  5. She can also mind control other merpeople and humans to so her bidding. She used this power to mind control Eric into marrying her, although it is unclear whether this was actually her power or the use of Ariel's voice, as the mind control was broken after Ariel's voice was restored to its rightful owner.