You have been warned.
FF1:
Based on Dissidia retcon, There are two worlds which are identical in many ways, but with some distinct exceptions. Some of these exceptions are what allow Garland to travel in time and become Chaos.
FF2:
There is an underworld known aptly as Hell, the capital city ruled by Satan is Pandaemonium. Long ago it surfaced and spilled monsters for the first time into the world. Powerful mages conjured up Ultima, a powerful spell which successfully assisted their pushing Pandaemonium back to the depths from which it came, and sealed it away. Fearing the spell's power, these same mages hid it away in the Mage nation of Mysidia. Centuries later, when the Emperor fights the Warriors of light, he perishes and his soul is sent to Hell. Unbenownst to the party, he was prepared for this eventuality, and managed to kill Satan to become the new Hell King. He then releases the city from its sealed status in Hell, and it surfaces as the final dungeon topside.
FF3:
The Crystal Tower acts as a connection to two realms: The forbidden city, known as Eureka, and The World of Darkness. When Xande is defeated at the top of the Crystal Tower, the Cloud of Darkness attempts to pull the warriors of Light into the other world for a final confrontation. This other world is made of mostly crystal and smoke, and like the World of Light, there are 4 special crystals and dark warriors. Based on the story, the World of Darkness has been twisted (and possibly rendered nonexistant) thanks to the Cloud of Darkness.
FF4:
In the underworld, there is a cave known as the Feymarch, known by the Dwarves nearby as “The world In-between”. This is the enigmatic realm of the Eidolons (Summons) and many vicious monsters. It is known that the passage of time in this realm occurs faster than that of the normal world, which explains when Rydia as a young girl is pulled into the realm by Leviathan, she is a matured woman when reunited with the party in a later battle.Also, deep in the Moon is the Lunar Subterrane, which is a crystal realm. Zemus/Zeromus refers to it as the Zemus-Zone.
FF5:
Four Meteors have crashed into the planet. At one stage of FFV, the party discovers that these meteors can be used as a portal to a n alternate version of their world. Furthermore, as the antagonist Exdeath is first defeated, he and his general Gilgamesh retreat to the Void realm, where the latter is imprisoned, and the former becomes Neo Exdeath. Gilgamesh’s connection to the Void has been used in the FF series as how he specifically is encountered in most of the series as the exact same person, with varying levels of skill/development, varying numbers of arms, and often accompanied by his loyal pet Enkidu, often portrayed as a treasure hunter seeking rare artifacts, such as Excalibur/Excalipoor. The variance suggests that while he may exist in many worlds of Final Fantasy, he may on each occasion exist as either younger or older form of himself, resulting in unusual, out-of place comments by this eccentric character.
FF6:
Beyond a sealed gate is the realm of the Espers, (a similar reference to that of Feymarch in FF4). As a break to tradition, the two worlds of FF6 are not divided by a void, but rather an apocalypse. The World of Balance thus becomes the World of Ruin, where the landscape is dramatically changed. In FFX|V: ARR, this sort of change already seems to exist, from existing concept. Side note: There are spells suxh as X-Zone and Dark Matter, which suggest a void realm where targets can be transported or banished, and destroyed. Kefka uses a skill like this to forceably convert Espers into their crystallized form known as Magicite, so that he can directly control and manipulate their powers.
FF7:
The Lifestream under the world of Gaia is being absorbed to power Midgar, a machine city, among others. This bares similarities to Aether and Ceruleum of FFXIV.
FF8:
The primary antagonist Ultimecia was attempting to compress time in an effort to become invincible, and to rule all existence. When Squall defeats her, the compressed time begins to unravel, and Squall finds himself trapped in a void made up of a barren, floating island, with clouds of gray surrounding in all directions. Eventually, he is rescued by Rinoa, but it has many visual similarities to the Void in other FF titles.
FF9:
There are two planets, Gaia, and Terra, which are effectively sharing the same space, except one is very ancient, and on the verge of dying. A major antagonist, Garland, is managing the absorption of Gaia into Terra, in an effort to continue Terra’s existence. His means of managing this is via a massive Lifa Tree, which is known as a conduit of souls, which pumps Mist into Gaia as a means to accelerate the end of normal Gaia life. Four corners of Gaia’s planet hold special connections to Terra, guarded by the elemental Fiends of Earth, Fire, Wind and Water. If special mirrors are placed in these locations at the same time, a place known as the Shimmering Isle will become a portal to Terra. This tunnel is known as the Void. Furthermore, when Terra is destroyed by Trance Kuja, he creates a realm known as Memoria above the Lifa Tree, which is made up of memories of each of the party’s memories. At the end of this realm is a barrier that the aprty crosses into the distant past, where the party witnesses the birth of Gaia, and eventually the crystal realm, and they encounter Trance Kuja attempting to destroy the original Crystal known to be the source of all life for Gaia and Terra alike. After his apparent success, the party is flung into a deeper part of the Void, known as the Hill of Despair, where they are forced to defeat the final Antagonist, known as Necron. Necron refers to the existence he/she/it wishes to create with no crystal and no life as Zero-World. (Note: Although Necron visually differs significantly from that of Neo Exdeath, Ultimecia and The Cloud of Darkness, they all bear similar agendas in regards to The Void).
FF10:
Specifically X-2: Energy from the underworld’s Farplane Is being absorbed to power Vegnagun, an ancient and powerful machine. This bares similarities to Aether and Ceruleum of FFXIV.
FF11:
The Void is portrayed as a realm of nothingness, seen as Promyvion & Apollyon. Furthermore, The major Antagonist Promathia is a key figure in the creation of an alternate realm known as Abyssea, which is effectively a mirror-Vanadiel, which ‘would have existed’ had Promathia not been defeated in CoP.
FF12:
Specifically, Revenant Wings: Fran's Quickening ability is known as "Gaze of the Void". While simple in name, it is designed to consume all active magical properties on any enemy targets.
FF13:
Specifically 13-2: Lying at the end of time is a realm known as the Void Beyond, where time no longer exists. The party, while traveling across the eons, occasionally finds themselves stranded in this realm. There is an alternate world, Known as Valhalla, which is supposedly the home of a deceased Goddess, and the ultimate resulting existence of the planet at the epilogue of FF 13-2. It is also the setting of Lighning’s seemingly unending battle with antagonist Caius.