The assumption is that the events of CoP open immediately upon the completion of RoZ. This is more or less confirmed with Esha'ntarl's rise to power (and Nag'molada's attempts to rise within Armathrwn Society) the after the disappearance of the Zilart Princes.
It goes kinda like this:
- Qufim is the original location of the Celestial Capital, Al'Taieu. This is supported by the proximity of Delkfutt's Tower, as well as the seemingly desolate areas of Qufim, Sauromugue and Batallia which show evidence of unnatural damage to the landscapes of each area. Its entirely likely that Al'Taieu itself was a landlocked mass before the Meltdown, leaving only Qufim in its wake.
- After the successful completion of the Chamber of Eventide, which cleanses a single soul of its Emptiness, Eald'narche touches the Mothercrystal hidden beneath the Celestial Capital. In coming into contact with the crystal, Eald'narche's Emptiness is totally washed away by the light of the crystal, and it gives the Zilart Princes an idea to return the world to its previous state of "Paradise".
- Tu'Lia is consequently built as a purification machine designed to cleanse the world of Emptiness in one go, using the light of the Mothercrystals - thus, opening the way to Paradise, which is why Tu'Lia is referred to as the "Gate of the Gods". The Arks that house the Mothercrystals and the Crystal Lines that connect them all were constructed at this time, with Delkfutt's Tower being the Control Center to direct the crystal energy.
- Tu'Lia itself was constructed OVER the Celestial Capital of Al'Taieu, but why? My own educated guess leads me to believe because of the presence of the incarnation of the Twilight God within the Celestial Capital, and that the incarnation was forming within the body of the King of the Zilart who is still unidentified at this time. It makes sense that the Zilart would want their capital cleared of the curse of Emptiness first, and follow up with the rest of the world second - it also makes sense that the incarnation would be drawn to the Emptiness contained within the Chamber of Eventide. Furthermore, it makes sense to use Delkfutt's Tower as a control center for the reason of its approximate location to Al'Taieu - it could also be designed to be a lookout tower to watch the results of Tu'Lia firing upon the Celestial Capital. This further suggests that Al'Taieu's former physical location was at today's Qufim Island.
- The Kuluu were adamantly against the construction of Tu'Lia, because it meant the literal destruction of their people, their way of life and their independent culture that they were beginning to form since the Zilart saw fit to exile them from the Celestial Capital (most likely by teleportation into the chamber where Diabolos resides today, the Shrouded Maw). They consequently made a Pact with the Terrestrial Avatars, who also saw their own doom with the completion and firing of Tu'Lia. We also know some Kuluu left Pso'Xja to create homes at today's Sarutabaruta and Elshimo regions.
- It's also important to note the role of the Dawnmaidens in all of this, particularly that of Yve'noile. The Dawnmaidens resided within the Hall of the Gods in Ro'Maeve, where the Zilart could easily travel from their Capital to worship both the Dawn Goddess and the Twilight God. Yve'noile herself, as we can probably all suspect and agree upon, is something of Altana's will or voice upon Vana'diel, a being even more "enlightened" than the Zilart. She was also against the Zilart Princes' plan, citing that the Emptiness was the will of the Gods. The Zilart Princes' continued their mission without her blessing. My guess is that after the Zilart Princes awaken in today's modern era, they modify the ruins of Ro'Maeve to be a functioning part of the Crystal Line to allow access to Tu'Lia, now that Yve'noile is no longer around to interfere.
So the big question is:
What actually happened during the Meltdown?
- Firstly, we know that Tu'Lia machine was attempted to be engaged by the Zilart Princes. Al'Taieu itself was consequently attacked on two fronts. In the skies above, Bahamut faced the defense system of the Celestial Capital. He fights valiantly, and prevails over the Zilart defenses. With this distraction in place, Selh'teus and Phoenix access Al'Taieu through the teleporter at Pso'Xja, likely using the energies of the remaining Terrestrial Avatars to force the teleporter within The Shrouded Maw to work in reverse (the machines behind the Avatar Gates). Selh'teus infiltrates the Chamber of Eventide, attempts to absorb the Emptiness of the Chamber into himself and dies. The Twilight God, sensing the death of the Emptiness contained near him, rages, and attacks Bahamut. Bahamut loses the resulting battle and falls into the Sea of Shu'Meyu.
- The Kuluu, agreeing that the Zilart must be stopped, arrange an attack on Fei'Yin, the closest Ark in proximity to their own greatest nation. Whatever defenses the Zilart placed there, if any, are overcome and Fei'Yin is destroyed. The resulting disruption of the flow of energy from the Vazhl Crystal creates an energy backlash within the Crystal Line.
- Meanwhile, Selh'teus is brought back to life by Phoenix, who presumably had to give her own life to revive him, but also thereby bestowing upon him the powers of Phoenix herself. Selh'teus essentially becomes the "next" Phoenix. However, this power alone is not enough to counteract the will of the Twilight God. Yve'noile then steps in to do Altana's will, binding the incarnation of Promathia into the royal audience chamber. She then sacrifices herself by entering the Chamber of Eventide to keep the remaining Emptiness stored there in check. Altana grants Yve'noile a blessing for her sacrifice, and the Celestial Capital is torn away from its physical plane.
- The backlash of energy within the Crystal Line has disastrous effects, and wipes out a good number of Zilart and Kuluu alike, while the light of the crystal shines upon lesser beings, bringing about the origins of the Five Races. This event is referred to as the Meltdown by Grav'iton.
- Centuries upon centuries pass, and we come to the event of the Multinational Expedition to the Northlands by Raogrimm and his party. Cornelia dies during the expedition, and the others return home.
- Raogrimm roams the Northlands in madness and comes into contact with the Deity of Darkness - there is speculation that this is the Celestial Avatar, Odin. He is granted power to sate his rage (sound familiar?), and hunts down and murders the surviving members of the Multinational Expedition. With the help of the demon army bestowed upon him, he constructs Castle Zvahl and becomes the Shadow Lord.
- Eald'narche and his brother awaken from their slumber due to a disruption in the energy of the Crystal Line caused by the presence of the Shadow Lord. They travel from the sea of Shu'meyu to Jeuno, a small fishing village, and using their knowledge of synthesis through the use of elemental crystals, they turn it into a sprawling metropolis.
- The Crystal War begins, is fought out, and ends. However, the distortions of the Shadow Lord's presence remain within the Crystal line.
- Age of Adventurers begins in the wake of the war. Kam'lanaut hires adventurers to study and fetch the crystallized Emptiness in the crystal lines, which he calls Magicite.
- The Shadow Lord is revived by some Dark Kindred followers, and is again defeated. Raogrimm comes to his senses, but the Zilart Princes show up and make sure the job is done properly this time. The power disruption fades when the brothers take the head of the Shadow Lord to reactivate Delkfutt's Tower.
- The events of RoZ happen here.
Eald'narche disposes of his brother and reactivates Tu'Lia. He ascends into the heavens to use the machine manually, and is defeated. However, he still attempts to fire Tu'Lia, and thanks to the influence of Lion, the firing is again disrupted.
- The events of CoP start here.
The second disruption of Tu'Lia leads to the awakening of Bahamut, who flees to Monarch Linn to gather together his army of wyrms to wage war. Selh'teus also uses this disruption to travel along the Crystal Line and form an avatar of himself within the physical plane (identified as his former boy-like form). His avatar begins its objective to siphon light from the Mothercrystals and place it into a living vessel to bring to Al'Taieu and revive the light of the fifth Mothercrystal to prevent Al'Taieu's fall back into the physical plane. His plan is to use his ability as the new Phoenix to revive the vessel after he replenishes the light of the Al'Taieu crystal.
- The events of Storms of Fate, Shadows of the Departed and Apocalypse Nigh all occur after these events.