Firelands

Firelands is one of the four domains of the Elemental Plane, occupied by the fire elementals.

Background
In the ages since the titans ordered Azeroth, the Firelands has served as a prison - not only for the mighty Firelord, Ragnaros, but also for the flame-born legions who serve him without question. The elementals had spent the innumerable years of their imprisonment reveling in the chaos innate to their being, until the Cataclysm ruptured the barriers between the Firelands and Azeroth. With the world still reeling from the disaster, Ragnaros and his minions have made it their goal to set aflame the World Tree Nordrassil, one of the remaining symbols of healing in the shattered lands of Azeroth.

Victory against the Firelord's servants is a possibility, however slim. While the druids of the Cenarion Circle and their allies work to establish a permanent foothold in the Firelands at the Molten Front, brave heroes fighting under the banner of the Avengers of Hyjal hope to push through the gates of Ragnaros's lair, Sulfuron Keep, and face off against the Firelord himself. Ragnaros bristles with unfathomable powers within the safety of his bastion, a far cry from his "borrowed" sanctuary in Blackrock Mountain's Molten Core. Here, surrounded by hand-picked followers and cradled amid the sweltering heat that boils forth from the ground and sky, he is an elemental lord in his prime. If Ragnaros cannot be defeated, then a conflagration unlike anything ever seen will erupt from the Firelands and engulf the whole of Azeroth in fire and ash for millennia to come.

For ages, the Elemental Plane served its purpose as a secure realm to imprison Azeroth's primordial spirits... until the Cataclysm ruptured its boundaries. Without warning, Ragnaros's armies surged toward Mount Hyjal, intent on burning the World Tree of Nordrassil to the ground. In the ensuing conflict, many brave heroes gave their lives to protect Hyjal from destruction. By their noble sacrifices, the impossible was achieved: the Guardians of Hyjal pushed Ragnaros's minions back into the Firelands.

Now, the battle to protect Hyjal rages in Ragnaros's smoldering realm. As territory is gained and Azeroth's champions edge closer to the Firelands' inner sanctums, a monumental task lies ahead. Entrenched around Ragnaros's lair -- Sulfuron Keep -- are his most trusted guardians, including the turncoat Druids of the Flame and their mysterious leader. Yet Hyjal's defenders cannot afford to shy away from any of these dangers. Should Ragnaros prevail against the incursion and succeed in destroying Nordrassil, Azeroth will suffer a blow from which it might never recover.

Ecology
The ash-choked Firelands is the least hospitable of the elemental domains. What serves as air in this harsh place is laden with scalding, noxious fumes. The only safe havens from the Firelands' ever-churning magma ocean are islands of jagged rock. Yet even the dangers of this merciless landscape pale in comparison to the raging fury of the elementals who dwell there. Despite this, the apparently uninhabitable land houses an ecosystem of its own, beyond elemental beings.

The fire spider brood of Beth'tilac sate their appetites by sucking the magma and fire from the corpses of elementals. They can be observed catching Molten giants and Fire hawks with their webs. The Hell hounds were the only things not being webbed or eaten by the spiders, but rather are trying to get at the spider eggs. The hellhounds can also be observed eating one of the Flamewaker trappers. The fire turtles are eating the scorpions. The local flora, which may either be native or imported and transformed by Druids of the Flame, appears to draw nutrition from molten lava.

World of Warcraft: Cataclysm
In World of Warcraft, the Firelands became accessible in the Cataclysm. Players can enter it through several flamegates found in Mount Hyjal, which Ragnaros and his minions have invaded. The places within the Firelands can roughly be divided in three categories:
 * Small zones intended for non-repeatable quests, accessible through different flamegates. Venturing through the gates and completing the quests on the other side is part of the quest progression in Mount Hyjal in which the player foils the invasion.
 * The Molten Front and its subzones, a new area introduced when the Guardians of Hyjal began their counterattack against the Firelord and his minions. It serves as a daily quest hub.
 * A raid instance simply called "Firelands", implemented in the major patch Rage of the Firelands (which also introduced the Molten Front). Just like the Molten Front, it also has several subzones. The raid consists of 7 bosses, and features both outdoor and indoor environments.

The Condensation of Electra-Atrocity, presumably written by, explains how the Twilight's Hammer clan maintains portals to the elemental planes.

RPG
Deep beneath the earth and seas of the Elemental Plane lurks the Firelands, a place of heat and ash. Although it is a harsh land, visitors can survive, for in one of those cruel paradoxes that define the universe, fire needs air to live and fuel to feed upon. Within the Firelands, these things exist in a constant cyclical state, with the very ash itself gradually transforming into fuel for future burnings.

In the center of a vast lake of fire stands Sulfuron Keep, the home of Ragnaros the Firelord. Until comparatively recently, he ruled his underground land with an iron fist. Three hundred years ago, he was summoned to fight in the dwarven civil war known as the War of the Three Hammers. This reckless act sundered the Redridge Mountains and created Blackrock Spire. Now, in a greatly weakened state, he lies at the bottom of that volcano on Azeroth, striving to find a way home. Meanwhile, in Sulfuron Keep and elsewhere across the Firelands, his elemental princes battle constantly for dominance.

The Firelands occupy the theoretical bottom of the "sphere" of the Elemental Plane. Just as the Skywall sits atop the plane, in a sort of bowl shape, the Firelands sit in the bottom of a similar curve. The "ground" underneath the surface is a sticky, bubbling mess of tar, ooze and rock fallen from Deepholm, which of course serves as the "sky." In some places, lakes of flammable oil and combustibles form, burning constantly and filling the air with superheated smoke. Organic and semi-organic material from Deepholm, and even the waters of the Abyssal Maw higher above, constantly filters down to provide new fuel for the endless flames. In places where the Fire Princes do battle with Therazane, huge volcanoes rise, spewing lava into the Firelands and upwards as well, sometimes reaching the floor of the Abyssal Maw or even the upper surface of Deepholm, where they blow noxious fumes into Al'Akir's realm.