The Waves of Calamity are mysterious phenomenon that occur every so often in the world and warranting the summoning of the Four Saint Heroes, to confront. The final arc reveals that the deity Medea Pideth Machina is responsible for causing the malignant phenomenon by merging together eight separate worlds. Her purpose is to drain the worlds of their energy for her own agenda; the suffering of the populaces is for her entertainment.
According to Glass the Waves are not a natural disaster, Naofumi and Trash together theorised that the one behind the Waves, an as of yet unknown god, does them intentionally to obtain an equally unknown benefit from them.
It is revealed later in the series that the Waves are an event where two worlds collide and begin bleeding into each other, thus the cracks in the sky through which monsters escape are in fact gaps opened between the two worlds. The waves can be slowed and stopped by allowing the Four Good Omens to complete their rampages, or they can be accelerated and completed by killing members of the Four Saint Heroes and Eight Vassel Heroes.
When the waves get to a very bad point, the two worlds are no longer able to maintain their separation and begin bleeding into one another, eventually leading to the destruction of one or both worlds. For example during the last wave period Glass notes her world has lost nearly all of its heroes and all of its Good Omens, while Melromark's has lost five heroes and three of the Omens. While Melromark's world is significantly better off and Glass' world is on the verge of total destruction, the skies of Melromark's world have begun to show not only cracks but vast shattered doorways into the other world, and landscapes from Glass' world have begun appearing in Melromark's, worse still the waves occuring turned from monthly events to frequent ones happening so often the heroes had to split up to confront them. Glass also notes that in the past they would see worlds that were utterly destroyed or lifeless appear, which are implied to be worlds where the heroes were utterly annihilated; it is in fact a viable option to save one's own world by destroying the other, which Glass herself attempted to achieve by trying to kill the Heroes of Melromark during the Third Wave.
Confronting a Wave
The waves usually occur on a monthly basis, however the deaths of the heroes can accelerate this while allowing the Four Good Omen's to rampage would slow it, if the heroes are wiped out then the world is destroyed and if the Omen's complete their rampages then the Waves stop completely.
When a wave occurs the Four Saint Heroes and their companions can be summoned directly to the location of the ensuing wave, except in the case of the Four Good Omen's Waves, which are unique in that the heroes are not meant to fight against them at all. Naofumi also found that adding a leader of another party, say a group of knights, as a member of your own party also transports him and his subordinates to the location of the wave as well, making it possible to transport a whole army towards the location, as proven by Trash bringing nearly all of Melromark's forces with him during the Tenth Wave.
While the heroes exist to hold off the waves and keep the worlds from completely merging (which would result in mutual annihilation), the Four Good Omen's work to stop the waves by killing vast numbers of living beings (approximately 66% of an entire planet's life forms) and storing up their souls.
The Four Good Omens/Four Sacred Beasts
The Four Heavenly Beasts, or good omens, are the Spirit Turtle, Houou, Kirin and Ouryuu. These beings have all been sealed away at present and were meant to escape during the 7th through 10th waves, though the actions of the heroes, Ren, Motoyasu and Itsuki woke the Spirit Turtle far earlier than planned. They cannot be sealed again during the last waves, meaning they must either be destroyed or allowed to fulfill their role as it is no longer possible to simply "survive" the Waves.
Once released they cause immense destruction, they pursue all living things, kill them and consume their souls. However, their purpose is not malicious, rather it is to confront the waves and save their world. By storing up souls they manage to push back the waves(the gap after the Spirit Turtle's rampage to Houou's is three months long due to the sheer number of people the former killed, compared to the standard one month gap) and they can effectively stop the waves in exchange for killing 2/3rds of the world's population.
They can be thought of as a last resort, saving the world itself at the cost of heavy living casualties, for this reason Naofumi chose to keep Dragon Emperor Ouryuu (sealed inside of Gaelion) alive until they approach the absolute worst case scenario.
Role of the Heroes
The heroes actively work to combat the waves when they occur. The Four Saint Heroes and Eight Vassel Heroes also act, by their very existence, to keep their world in place and prevent it from merging with the other world. None of the previous generations of the heroes have ever actually managed to defeat the waves, rather they have all just managed to survive them instead, which is not possible during the last wave period.
Glass' world, which also has Four Saint Heroes different from the Sword, Bow, Shield and Spear, is another example, and it is the world that Melromark's is fusing with in the last waves. Glass notes that in her world the heroes are nearly all dead, and the world itself is as such falling apart, which mandated her going over to Melromark's world to attempt to kill all of its heroes, thus saving her own world by destroying the world it is fusing with. However, after realising that Melromark's world still has Ouryuu and several heroes, Glass proposes that she transfer people from her world over and they instead destroy her world (which is already effectively dying on its own) in order to save Melromark's.
The One Behind the Waves
The prime instigator of the waves is Medea Pideth Machina , who performs them to drain each world of its energy for her own agenda; the suffering of the populaces is for her entertainment. Initially it was postulated that Medea either cannot interfere directly with the worlds or instead simply chooses not to, but it has been noted that her patience has limits and they will attempt to make a direct assault if pushed into it. However this was proven wrong as Medea demonstrated she could easily destroy the world and collect the remaining energy immediately after manifesting but decides to prolong the campaign so she can enjoy monitoring her pawns as they incite war. Medea has the ability to reincarnate souls by using fragments of her soul, which she uses to create people such as Tact or Malty into the various worlds she is fusing by the waves to disrupt or eliminate the heroes who are thwarting her machinations.
Though why is unknown, the waves are not naturally disasters, they are intentional acts caused by an as of yet unknown God for an equally mysterious gain.
Originally the Four Saint Heroes and the Eight Vassal Heroes(Two Vassals were made to serve each Saint) came from completely different worlds, which ultimately were destroyed/absorbed into other worlds by the waves. The Bow Hero and Shield Hero came from the world where the Demi-humans/Beastmen originated while the Spear and Sword Heroes came from the world where humans lived. The Bow Hero managed to make himself well received by the humans while the Shield Hero remained a figure of contempt for them up to the time where Shield Hero Naofumi Iwatani was summoned.
In a similar vain Glass' world also has Four Saint Heroes and Eight Vassel Heroes separate from those of Melromark, they also have three races, Humans, Demi-humans(in their case, dwarves, elves and the like, as oposed to the beastmen and animal-like Demi-humans of Melromark's world) Crystal people and Ghost kin, all of whom are assumed to have imigrated from other worlds during past waves.
The Wave period in which the Four Saint Heroes Naofumi, Motoyasu, Itsuki and Ren are summoned is noted to be the very last one, meaning that if the world is not saved here it will ultimately be destroyed. The worlds that are being fused in this case are Glass' world and Melromark's world. The waves listed below are the main ones seen in story, however several other waves are noted to occur over the globe during this period which are handled frequently by other heroes.
Pre-Sacred Beast Waves
The waves taking place before the Four Sacred Beasts were released to stop the wave. These waves are typical, with the sky splitting as two worlds collide and monsters coming out. They have a stable month gap between their onsets and the monsters are weak enough to be handled by a proper military response, even some adventurers are shown to be able to hold their own against monsters of these waves.
Struck at what would later become Rock Valley in Melromark, this Wave killed Eclair's father, both of Raphtalia's parents, and rendered several demi-humans homeless and/or at the mercy of slavers, this is not helped by the King of Melromark's hate of demi-humans and the queen's unfortunately timed absence. This event led to the king summoning the Four Saint Heroes.
Struck near a small town in Melromark. The casualties were severely reduced due to the quick response that the knights could provide due to the distance from the Castle Town being short and the heroes Motoyasu, Itsuki and Ren managed to easily slay the Chimera acting as the boss. Meanwhile the Shield Hero Naofumi directly protected the nearby town and saved countless lives in the process but would go unrewarded by Melromark's King for his contribution due to favouritism.
The Third wave occured near a number of villages in Melromark's northern region. During this wave the Shield Hero Naofumi managed to bring several knights with him at the time of teleportation, which was good because the distance from Melromark's Castle Town meant it would take a day and a half for the knights to arrive normally. Together with the knights and some adventurers, as well as a certain old woman who happened to be a retired Lv.100 adventurer, Naofumi managed to defend these towns. After three hours however the Wave still hadn't ended and Naofumi was forced to investigate why the other three heroes hadn't stopped it yet and he finds them defeated by Glass, who herself had already killed the wave's boss. Naofumi engages and defeats Glass together with his party, forcing her to retreat while warning him not to believe the waves are just a natural disaster.
Glass later notes her intention was to destroy Melromark's world by killing off its heroes in order to save her own world, something Naofumi notes he can't argue with as he'd likely have done the same thing in her position.
Four Sacred Beast Waves
The Four Sacred Beast's waves are destinct from those before for a number of reasons. The Beast's primary purpose is in fact to supress the Waves and protect the world by killing 2/3rd's of the population on it and absorbing their souls. They aren't meant to be fought, thus the Four Heroes aren't summoned to their location and aren't supposed to fight them at all. They can ultimately stop the waves if left to do their job, but will, as mentioned above, accomplish it by killing off millions of living beings. The more they kill during their active periods the more time lasts until the next wave, for example the Spirit Turtle managed to kill thousands of people during its rampage and as a result Houou's wave occured three full months later, however Houou didn't kill many people by comparison and this caused the wave of Kirin to occur only three days later.
The 7th wave occurs when the heroes Motoyasu, Itsuki, Ren and their parties foolishly release the Spirit Turtle from its seal, effectively skipping any waves that should've occured prior to this one and pushing this one forward. The fourth hero, Naofumi, together with an allied force led by Melromark's Queen, confront the Spirit Turtle in a bid to seal it anew, but find that it is impossible to seal the beast. Naofumi interprets the messages of a previous hero, one named Keiji who helped seal the turtle to start with, that the turtle can only be killed by destroying both its head and heart at the same time, which he proceeds to do just as it started storming the Castle Town. This incident itself took roughly a full month to resolve.
Spirit Turtle Familiars
The Eigth Wave occurs 3 months after the 7th, and incurs the revival of Houou. The allied forces and Four Heroes mounted a massive force to confront Houou, who revives in the form of two Pheonix that must be simultaneously slain(should one go down before the other, it'll self destruct and both will revive from its ashes). The allied forces nearly defeated the two Houou without suffering heavy losses, but the sudden interuption of a certain individual killed one of the Houou too early and triggered the second to self-destruct, killing a large number of the allied force, including Atlas. Atlas' sacrifice allows Fohl to awaken the Seven Star Gauntlets and with her dying words she encourages Naofumi to unlock the Shield of Compassion, which allows him to vastly increase the power of all slaves, demi-humans and companions forming the allied force with sudden growth corrections. The allied forces swiftly slay both Houou with this newfound power and begin hunting down the one who interrupted and caused the deaths of so many of their comrades.
Occurs 3 days after the 8th wave in the kingdom of Folbley and comprises of Kirin's unsealing. This wave is resolved by Tact and his army soon after. Naofumi theorises that the reason this wave struck so fast is either due to the swiftness of Houou's supression preventing casualties, or Tact's senseless murdering of four seven Star Heroes sometime prior to the 8th wave.
The blue hourglass summoned by the Spirit Turtle dissapears after Kirin's demise and the tenth and last wave is depicted with a normal red hourglass, which shows that it will occur in 1 week.
Post Sacred Beast Waves
The waves occuring after the Four Sacred Beast's waves are far more severe than the one's before them and far, far more frequent due to the fact that Tact killed off five of the Vassel Heroes(also due to the fact that Glass' world has lost nearly all of its own heroes). The average monster's strength spikes to around Lv.220, the cracks in the sky vividly depict the landscape of the other world and, following the wave's onset, the landscape of that other world becomes a part of Melromark's, the waves also occur on a regular basis, the Tenth Wave occured only a week after Kirin was supressed and the waves after it are so frequent that the heroes had to split up into four teams of three(One Saint Hero and Two Vassel Heroes each) to deal with it.
The situation at this time becomes so bad for Glass' world that she decides to bring people from said world over to Melromark's and then have her own world destroyed to save it, something she has the backing of Melromark's world's leaders and all of the heroes to do.
Was likely supposed to incur the summoning of Dragon Emperor Ouryuu, but he was presently, and willingly, sealed in the form of the Dragon Emperor Fragments.
The enemies this time around are Lv.220 and are insanely powerful. Glass appeared at this time, once again trying to kill the heroes of Melromark to balance with her world, but the situation was so overwhelmingly bad that she found herself teaming up with Motoyasu, Fitoria and Itsuki to hold off the monsters. When Naofumi and Trash arrive with their reinforcements to stop the wave they attack the Wave itself, which is quickly expanding, and destroy it, but the landscape before their eyes is completely transformed into one of Glass' world.
In the aftermath of this wave Glass asks for permission to bring people from her world over to Melromark's, after which they will destroy her world to save it. Several waves of this sort also start spiking up after it, but the heroes manage to supress them by splitting up.
- The 7-10th waves are spearheaded by the Four Sacred Beasts: the Spirit Turtle, Houou, Qilin and Ouryuu repectively.