Hell is the place where Gielinorians on Santa's naughty list go when they die. However, there are some natural inhabitants of it (pretty much everybody you encounter in SFCII).
It was run by Zamorak until you killed him. Afterwards, it was left alone.
After the prologue, Screenshot Fake CYOA 2 took place in Hell. You and the C journeyed there to kill Zamorak and avenge Guthix.
The path to Zamorak's fortress in Hell is guarded by the demon bodyguards.