The recurring wars between Cosmos, the goddess of Harmony, and Chaos, the god of Discord, are fought in cycles in Dissidia Final Fantasy and Dissidia 012 Final Fantasy. A new cycle of war is introduced in Dissidia Final Fantasy NT.
Unbeknownst to the two factions, the wars are masterminded by Shinryu and Cid of the Lufaine, so that each time a faction wins the survivors are killed and everyone is revived to begin a new war. Cid wanted to refine Chaos into the ultimate weapon to take back to his original homeworld and seek vengeance on Onrac, the government that destroyed his family, while Shinryu would absorb the warriors' memories and experiences and become stronger.
There have been twenty cycles seen or alluded to in the two games.
1st to 11th cycles
Cosmos and her warriors lost every cycle. During the second cycle, three warriors could not withstand purification, and were not summoned again. It is unknown who they were. The Warrior of Light appeared in an early cycle, Shantotto and Gabranth left the world through portals to the Rift, and Prishe disappeared forever after Garland defeated Cosmos. It is unknown if this is the second cycle, or a different one.
The main story of Dissidia 012 Final Fantasy, where many of Cosmos's warriors fail to stop the infestation of manikins, which is why Shinryu does not revive them in the next cycle. At this time, participants from both sides were transferred to the opposing group.
The main story of Dissidia Final Fantasy, as well as the bonus chapter of Dissidia 012 Final Fantasy. In this cycle, several Chaos and Cosmos warriors have switched sides while Cid feels guilty and has rebelled against Shinryu. The Cosmos side wins in the end and the heroes return to their respective worlds by Cid possessing Chaos to teleport them away.
End of the cycle
The thirteenth is the final cycle, as supported by the secret reports and character info on Cosmos, Shinryu, Cid, and Chaos. Shinryu absorbed Chaos's experiences for its own and fled to the Rift. Cid of the Lufaine, released from his prison in Scenario 000, reunites with Cosmos (revived by the power of the warriors' crystals).
Nightmare world cycles
The following cycles take place in the nightmare world Cid is placed in during the thirteenth cycle.
14th to 18th cycles
Because of Cid's actions, the pact with Shinryu was broken and the dragon sought vengeance for being robbed of the power, experiences and memories of the summoned warriors. Shinryu forged a pact with Chaos, and sealed Cid in a nightmarish alternate world where the war didn't end in the thirteenth cycle, Chaos survived, and the war continued until the twentieth cycle.
In this reality, Cosmos has never won and Chaos's power continues to grow from the discord and destruction of the endless war. After the eighteenth cycle, Chaos is overcome by the power. His mind breaks and he descends into insanity, becoming Feral Chaos. He wipes out all of the Warriors of Cosmos at the beginning of the cycle, and turns on his own warriors in his madness.
The Warrior of Light, the only warrior born in World B, met his death at the hands of Feral Chaos, being the last warrior standing in protection of Cosmos.
Shinryu has accumulated enough knowledge and strength from the wars and evolved into his final form, called Shinryu Verus. Little else is known about this cycle.
At the start of the twentieth cycle, Feral Chaos attempted to slay Cosmos who had lost the power to summon warriors. Cid took the form of a moogle and rescued her from Order's Sanctuary, sealing her in the Cavern of the Earth near Melmond, where Chaos would be unable to reach her. Denied his victim, Feral Chaos challenges Shinryu. Cid seals the northern continent to contain the conflict between Shinryu and Chaos and seals himself into the Cavern of Earth with Cosmos.
It is implied the Confessions of the Creator scenario is the aftermath of the twentieth cycle. The world is devoid of life apart from moogles and the manikins, although there exist specters of long since defeated Warriors of Cosmos. When the player finds Cid, as the "mured moogle", he is alone, and it is unknown what happened to Cosmos. In musing upon how the player party arrived in the nightmare reality, Cid theorizes they were unintentionally summoned by him, as neither Chaos nor Cosmos is in a position to summon warriors. Once Feral Chaos is defeated, he regains his sanity and thanks the player party for freeing his father, and the nightmare reality dissipates.
The following is a list of the warriors and the cycles they participated. Though no cycles before the twelfth are named, the knowledge that the more someone fights, the more memories they regain, allows an approximation of when the warriors likely appeared in the cycle by judging how much they remember of their own worlds. The warriors are listed in chronological order of their original games in the Final Fantasy series.
The Warrior of Light's cycle of entry is unknown, but was an early one. Garland was present in the cycle from the beginning, being summoned by Cid to serve as Chaos's right hand.
Firion may or may not have participated in cycles before the twelfth, as he has few memories and needs the wild rose to stir them. If Firion participated in earlier cycles, it was not many, for his memories are sparse. The Emperor has been in many cycles, including at least one where Garland, Gabranth, Shantotto, and Prishe were present, as seen in an official quest.
The Onion Knight's cycle of entry, like Firion's, was likely just before the twelfth cycle or the twelfth cycle itself, as he cannot even remember his own name. It is unknown when the Cloud of Darkness entered the cycle, as she indicates to know the "legend" of the Onion Knight warrior in the thirteenth, but no indications of the lengths of her memory are given otherwise.
Cecil appears to have been summoned for the first time in the twelfth cycle, but regained his memories of Kain and Golbez during it, although he simply perceives Golbez as an enemy and is quick to attack him because of it, in large part due to not remembering their relationship as brothers. However, he seems to regain that memory, as well, either before or during the thirteenth cycle. Kain is stated in Yuna's museum profile to have participated in the cycles as long as she and Jecht have, indicating he has been through many cycles, and has thus regained many of his memories of home. Golbez remembers Cecil and Kain and has other memories of their home, but it is not made clear if he remembers as much as Kain.
Bartz's museum profile states that he does not have vivid memories of his past, but he remembers Galuf, though not by name, so he likely participated in at least one cycle before the twelfth. Exdeath's museum profile states he has recovered most of his memories, including those of Bartz and Gilgamesh, so he has fought in many cycles. Gilgamesh, as an outsider to the cycles, may have stumbled into them countless times over their course and has thus maintained all of his memories since he was not summoned by either god. He was present in at least one early cycle in which he sparred with Prishe.
Terra likely participated in a cycle prior to the twelfth, as she knows about airships during that cycle, and in the thirteenth cycle, she tells Cecil that she has some memories of war and battle from her world. Kefka could have taken part in any number of cycles, as there are no indications of any memories he has or has not recovered. Kefka had enslaved Terra arguably before the twelfth cycle, knowing of what she is capable, so at the very least, he remembers her from their world.
Cloud took part in several cycles, having regained many of his memories. Tifa was summoned for the first and only time in the twelfth cycle, so she has not remembered anything except for her name and her memories of Zangan. It is ambiguous as to which cycles, if any, Sephiroth participated in prior to the twelfth, as he does not fully remember Cloud or Tifa, and he only remembers the lifestream and Jenova after fighting the two. He thus has few memories of home, and so it is likely that he only took part in a handful of cycles prior to the twelfth.
Squall was present in at least one cycle prior to the twelfth, as in an official quest during that cycle, Cloud alludes to the two of them meeting in a past cycle. Squall is aware in the thirteenth cycle that Ultimecia is his destined opponent, indicating that he remembers her from his world. He also remembers Rinoa, though not by name, toward the end of that cycle. It is unknown how many cycles Laguna participated in, as he doesn't remember Squall or indicate any other memories of home. It is unlikely he would remember Squall, however, no matter how long he fought, as this Laguna appears to be from the past, as seen in the dream sequences in Final Fantasy VIII, and thus would not know Squall. However, his pre-battle interaction with the Onion Knight hints that he might remember Ellone, given that he calls himself "Uncle Laguna". Ultimecia has been through "many" cycles, as stated in her museum profile.
Zidane is stated in his museum profile to have regained many of his memories, so he has taken part in several cycles. The same can be said of Kuja, who, by the time of the twelfth cycle, is bored with the fighting, and the two are initially on friendly terms (prior to Kefka meddling with Kuja's memories), showing that they remember each other well (and likely mended fences at the end of the events of Final Fantasy IX).
Tidus was a new warrior in the twelfth cycle, and thus had no memories of home, save for his name and his hatred of Jecht. However, he regained more of his memories either before or during the thirteenth cycle. Yuna and Jecht have both been through many cycles and remember much about their home world, including each other, Tidus, and Yuna's pilgrimage. However, Yuna tells Tidus that she doesn't remember everything yet.
Shantotto and Prishe were summoned in one of the earliest cycles. It is ambiguous as to which specific cycle they vanished, but during their time in the war, they regained many of their memories of each other and their home world, with Prishe even remembering her pet name for Shantotto.
Vaan may have taken part in cycles prior to the twelfth, or may have been newly summoned during that cycle. He indicates several times that he is fascinated with airships, but there are no hints of any other memories that he has recovered. Gabranth was one of the earlier participants, and although the exact cycle is unknown, he vanished from the war around the same approximate time as Shantotto and Prishe. In a battle with Shantotto, he alludes to fighting her in a previous cycle, so Gabranth has been through at least two cycles. In an official quest where the two meet, neither Vaan nor Gabranth indicate that they know the other, but as Gabranth was dismissive of Vaan's presence in Final Fantasy XII, he may simply not have acknowledged remembering him. Vaan's pre-battle interaction against Gabranth indicates a sheer, angry determination to defeat him, perhaps alluding to his will to avenge the murder of Reks. In this case, Vaan may at least remember Gabranth.
Lightning has few memories of home and has them stirred by the presence of Firion's wild rose. During the twelfth cycle, she uses the term "Primarch" when speaking to the Warrior of Light, indicating that she remembers the Sanctum. When she battles Exdeath, she recovers her memory of l'Cie and fal'Cie, but does not use the specific terms. She may be newly summoned in the twelfth cycle or have only taken part in a few cycles prior. In Dissidia Duodecim Prologus Final Fantasy, Lightning narrates to the player about herself being a servant of the gods, "That's what I found myself, when the goddess of harmony called upon me." It is unknown whether she was narrating about the twelfth cycle only, or narrating about the greater conflict in general. If the former is the case, then she was summoned for the first and only time in the twelfth cycle. If the latter is the case, then she has been through at least one cycle prior to the twelfth.
In addition to the above, Report 14 mentions a warrior in an early cycle who possessed a powerful mind that puzzled Cid, leaving him to wonder if the warrior was connected to Shinryu. At the end of an early cycle, three warriors were so severely weakened, they perished upon being purified by Shinryu. It is unknown who these unnamed warriors were, or which god they served. This could refer to the cycle in which Prishe vanished when Cosmos was killed, but it is uncertain.
Locke, Rinoa, Kam'lanaut, Snow, Y'shtola, and Noctis, all introduced in Dissidia Final Fantasy NT, are known to not have participated in the old conflict. The statuses of Vayne, Zenos, Ardyn, Ramza, and Ace are unknown, though all are presumed to also be newcomers to the conflict.