Hell is the afterlife plane of existence where the souls of the damned are sent to spent eternity with torment and punishment based on their actions from their previous life. It is based and evidenced on folkoric and religious traditions, where it is told as the final destination of a person's life, alongside with Heaven, Paradise, and Purgatory. Hell has served as influence and inspiration for multiple fiction works including Dante Alighieri's Divine Comedy play, as well as the video game and resulted anime film adaptation, Dante's Inferno, based on Dante's play. In the latter case, the titular protagonist surveys through the hellish landscape in hopes of defeating his demons, reuniting with his true love, and escaping from the clutches of Lucifer.
Regarding the villain tournaments, Hell appears as one of the non-mortal planes of existence, alongside with Limbo, the Spirit Realm, and the Netherworld, with the use of Dante's Inferno animated film being used to represent its existence. There is also the footage of Hell from the Dragon Ball franchise used to represent it's existance in many cases.
Chernabog has been portrayed as the ruler of Hell in many tournaments. However Malmoth is the ruler of Hell in Worst Villains Tournament Ever and Worst Heroes and Villains War Ever. The same goes for the Nightmare King, who rules Hell in Smash Bros Lawl Infinite.