The Netherworld is Marvel Comics attempt at the definement of "Hell", complete with its own ruler Mephisto. Same fire, brimstone, little demons, screaming souls, etc. Whenever an evil character dies, he always ends up there. The Netherworld can be breached by only the most powerful of sorcerers (such as Dr Strange) and the lost souls can be rescued on occasion.
Currently the Netherworld is ruled by the Demoinic entity known as Blackheart
, who after killing his father Mephisto, usurped the power of the Netherworld for himself and became the current ruler.
"The Netherworld isn't a place you'd want to live.... hell you wouldn't even want to visit! Oops, I said Hell, no pun intended, really!"