Sukuna Ryoumen

Known as the undisputed King of Curses (呪いの王), Sukuna, according to legend, was a demon (though in truth was a human Sorcerer) during the prime era of Sorcery 1000 years ago, and Sorcerers gave their all to defeat him. After his death, his corpse was unable to be fully destroyed, and thenceforth his remains of 20 fingers, preserved in grave wax, traversed the ages as Cursed Objects. In the present, Sukuna was incarnated into Yuuji Itadori due to the latter eating one of his cursed fingers, which contain his fragmented power.

(Source: Wikipedia)