Thread:ShamelessNitpicker/@comment-7931550-20171122093630/@comment-30053419-20171122111428

Well, the Hell-Hole isn't literally a portal to hell - that we know of. Clem speculates that something within the island's soil (or the afflicted area) has turned "poisonous and vile." Also note that the Malefactors emerge in a variety of ways: according to Clem, the leap from the Hellhole is supposedly unique to the Slayers.

The Malefactors and the ghosts associated with them are derived from actroctities committed within a certain area - sometimes karmic punishments inflicted on the perpetrators of these atrocities. We can confirm that the phenomena is native to Earth, that the events or individuals that initiated them are native to Earth; we don't have any in-universe data on any other possible origin.