Following Zaros' theft of Saradomin's Crown/the Locator and his declaration of intent to claim the mysterious monolith/the Codex, a war breaks out, taking place over four battles.
Battle One
The Zarosian army- Azzanadra, Nex, Char, the Temple Knights and some Abyssal Prowlers- assault the Archaeology Guild. They are faced by an alliance consisting of the World Guardian, Saradomin, Seren, Crux Eqal and their forces, including Thaerisk, Helwyr and Zilyana. The forces are evenly matched, and it seems that there won't be a decisive victory for either side soon.
Battle Two
Wahisietel, who's staying on the sidelines, comments that the Zarosians seem to have underestimated the power of the alliance. Of course, that's when something happens to change everything: Zaros teleports in, commenting that he has to do everything himself. Seren teleports in and blocks his path to the Codex, and tells him that whatever his plans are, they have to end there. Zaros tells her that he wants the Codex, and Seren asks him if his greed is endless, or if the Codex will sate it. Zaros tells her to mind her own business and go back to planning her garden, even though she knows as well as him that Bik won't give a damn about it- but at least he won't be there to see her regret it.
Seren asks why he's leaving when he could stay and help her save the world and their family. Zaros says that Gielinor is lost and the Elder Gods were never their family. Seren says they owe the Elder Gods their existence; Zaros says he owes them nothing and tells her to get out of his way. Seren says he can't order her around and she has the power to stop him, but Zaros calls her bluff, saying that she'll have to either kill him or move. Seren gives in and leaves, and Zaros has the Codex.
Azzanadra asks why Zaros is leaving. Zaros explains that the Codex wasn't actually made to hold information, it was made to seal a crack in reality that leads to another location, one that Azzanadra identifies as Erebus, the Zarosian afterlife. Zaros is seeking power, so he's going to Erebus to find it. Azzanadra wants to go with him, but he can't- Zaros says it would kill him. Zaros forces open the Codex and enters Erebus, but he doesn't close it, and wave after wave of Shadow Cacklers emerges and attacks.
Azzanadra offers a truce with the alliance, because they'll need everyone to handle the shadow monsters so they can close the Codex. Saradomin is reluctant because the Zarosians caused the whole mess in the first place, but finally agrees, and everyone does what they can to defend Gielinor.
Battle Three
Things are getting worse: the waves of shadow monsters aren't ending, and they can't get past them to close the portal. They need reinforcements, and the World Guardian suggests the TokHaar, who are powerful warriors and were made by the Elder Gods to defend Gielinor. When asked, TokHaar Hok doesn't provide a real answer, but soon the TokHaar arrive to help the alliance.
Unfortunately, the Shadow Cacklers have pushed past the alliance's defences and are trying to attack both the wounded and the gates of Varrock. However, with the TokHaar's help, the alliance manages to repulse the invaders...
Battle Four
...so naturally, that's when things get worse. Char makes the extremely stupid decision of using a power-enhancing artefact while standing next to a portal to an incredibly dangerous and unknown world, and promptly gets sucked into Erebus. Azzanadra decides that enough is enough and closes the portal, but the backlash turns him into a god. The fight against the monsters from Erebus is over, but the TokHaar survey the devastation and decide that the damage inflicted on their world was unacceptable, so they'll have to kill everyone and start over. They march on the monolith and Varrock's gates, but the alliance manages to repel them.
TokHaar Hok sees how many of his people have been killed and calls for a retreat, saying they need to return to the lava because the Elder Gods will need them. Saradomin says that his people will guard the monolith, because now that they know that it's actually a portal, they need to make sure that it stays closed. Azzanadra says that they don't need to worry, it'll stay closed, but Saradomin says that he's done enough and he should leave now.
Speaking to various people afterwards gets the World Guardian some new data: Azzandra expects Zaros to return eventually; Nex is furious that Char is gone and is determined to get her back; and Wahisietel and Trindine reveal that Acting Guildmaster Reiniger has discovered an intact area of Senntisten to the north. (After the quest, Owen will also tell the World Guardian that he and Saradomin went on a quest to see if they could return the centaurs to life. They couldn't bring them back, but they helped the centaurs' spirits move on to the afterlife, though the Wand of Resurrection crumbled to dust.)
Unfortunately, things have got worse: opening the portal to Erebus released a wave of shadow anima, and the eggs, empowered by that anima, are starting to hatch...
