Whom did Krishna cursed to live forever?

Krishna cursed the demon king Banasura to live forever after he was defeated in battle. Banasura had been granted a boon of immortality by Lord Shiva, but Krishna decreed that he would live eternally in a form that would suffer without the ability to die, ultimately becoming a guardian of the universe. This curse was a way to ensure that Banasura would be kept in check and serve a purpose in the cosmic order.