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Hell was a place mentioned until the end of Warcraft II and Lord of the Clans. It was a place of suffering and the adobe of the daemons. As of today, it's considered to Retcon speculation. Hell probably has little to do with the Twisting Nether, or Firelands, as the Firelands is a place among the Elemental Plane. Hell itself would have more to do with the Twisting Nether, which is described as Hades or the Underworld. Hell could also be a different place from the Twisting Nether in where it only exists in an afterlife, stating that the Twisting Nether which is like hell where a Satanic figure responsible for the burning legion would reside in until being
In early versions, Hell and the Twisting Nether seem like different places. Another name used was the Dark Below. Normally Hell is a place of Torment among disbelievers and wrongdoers while the Twisting Nether by contrast seems like a place of Oblivion or Astral dimensions, not necessarily a place of suffering. Practically, in Hell, those who disbelieve in God and associate with Idolatry (as the up-most majority of races in the war-craft universe do) will be sent to the burning fire for all eternity after the day of judgement (also lacking in the majority of the folklore in the race's of warcraft).
Another possibility is to assume that the Dark Below of the Twisting Nether is the home of a Satanic figure with his Satanic army in the WoW Universe who is the originator of all evil, as such beings did corrupt Sargeras. Therefore Hell would be a separate place of torment for Satan himself, as well as wrongdoers, non-believers and idolaters, after there lives end and are resurrected to be judged (also very lacking in common lore), as Satan himself is punished in hell, while some sources say Angels would rule Hell in the place of the wrongdoers to throw evil back against them. Other's say Hell is a mistake or something not done by God, to awnser the problem of evil, maybe a reason why many have turned to others such as the old gods or titans for a deity of sustainers to replace the creator of the Universe.