The Weight of Three Crowns

The Vestige has convinced the leaders of the three alliances to meet to discuss a plan to attack Molag Bal's realm of Coldharbour. The meeting takes place on the historic island of Stirk. Hopefully, the meeting will go smoothly.

Quick walkthrough
1. Talk to Vanus Galerion. 2. Follow Vanus through the Portal. 3. Wait for the leaders to approach the negotiating table. 4. Talk to Vanus and Hakruba about an alternate plan. 5. Convince the alliance leaders to agree to the new plan. 6. Talk to Vanus. 7. Repel the invaders. 8. Lead the army through the portal into Coldharbour.

Full Walkthrough
After completing Messages Across Tamriel, Vanus Galerion will be inside the Fighters Guildhall of your alliance capital, which is either Mournhold for Pact players, Wayrest for Covenant players and Elden Root for Dominion players. He tells you that everything is ready for the summit at Stirk. Once you inform him you are ready, enter the portal he opens. You, Galerion and Countess Hakruba of the Fighters Guild, will all arrive at the northwestern edge of the ruins where representatives from each of the three alliances have gathered. Vanus has assembled a team of experts on Coldharbour and he asks you to meet them on the way to the negotiation table. The summit takes place at a table in the center of the island, southeast of where you arrived on Stirk. Galerion will greet each expert as you make your way up the path. The experts include Lady Clarisse Laurent, Telenger the Artificer and Captain Alphuary.

Negotiations Begin
Once you climb the stairs to the meeting point, High King Emeric will approach the table from the left, where the Daggerfall Covenant camp is. Then, Queen Ayrenn exits from her portal across from you. Finally, Jorunn the Skald-King appears between the Ebonheart Pact banners on the right. Ayrenn speaks first asking the other rulers to put aside their differences. However, King Emeric questions the meeting and asks if it is a ruse, a plan to divert Daggerfall and Ebonheart forces while the Dominion takes the Imperial City. Jorunn adds that he thought backstabbing was Ayrenn's favorite tactic. Ayrenn responds to both by replying that Molag Bal is more powerful than either of the other two alliances. Emeric refuses to help because he says he will not endanger his kingdom based on the words of Jorunn and Ayrenn.

An Alternate Plan
Vanus interrupts their arguing, but all of the rulers have left the negotiations. You suggest to Vanus and Countess Hakruba that the guilds lead the attack. Vanus suggests you visit each leader individually to propose the idea, while Hakruba reminds you of the non-interference pact that binds the guilds. In order to fight Molag Bal's forces across Tamriel, each leader must accept the guilds' presence in his or her realm. All three leaders must consent to the new plan.

Convincing the Alliance Leaders
Both Emeric and Ayrenn disagree with Jorunn's stubbornness. However, you persuade Emeric and intimidate Ayrenn, who responds that she cares about her people and that her legacy will be one of a bold and decisive leader. Talking to Jorunn reveals that he tried to get the other leaders to co-operate. Emeric believes Ayrenn has no right to impose her will on all of Tamriel, but after some persuasion he agrees to consider the plan. King Jorunn refuses to be swayed by an "Altmer lapdog" until you intimidate him by questioning his legacy as the king who surrendered to Molag Bal. Queen Ayrenn is hesitant about sending the guilds against an unknown enemy but agrees because the other alliances will never unite and stop Molag Bal otherwise.

Molag Bal's Forces Assault Stirk
You tell Vanus, who has now moved to the table, of the agreement. However, he senses something is wrong as the air suddenly feels heavier, as if a storm is approaching. Caldien, who has been announcing each of the rulers, suddenly drops to one knee and says, "He has arrived." He pleads for Vanus to get away from him and run, before he is struck by lightning, and Molag Bal appears behind him. Molag Bal plans to destroy all three of the alliances at once because their leaders are all in one place. He announces his plan to rule all of Nirn and disappears, killing Caldien and causing Daedra to spill out of newly-opened rifts. The island is under attack from Daedra pouring out of rifts around the island. Galerion will prevent more rifts from opening, while Countess Hakruba protects the leaders and you close the unstable rifts. In the Ebonheart Pact area, the Skald-King is under attack from a great Daedric Titan called Sormorask. Ra'Lorka, a harvester, is attacking Queen Ayrenn in the Dominion camp, and Bitterwind, a storm atronach, attacks King Emeric. You return to Vanus who fears there is no time to waste. He and Hakruba confront the leaders in a speech asking them to trust the guilds and sanction their actions. Emeric and Jorunn agree, and Ayrenn remarks that much of their distrust was sown by Molag Bal.

The Battle of Coldharbour Begins
Once you successfully repel the Daedra, Vanus announces that the time has come, and you follow him into a valley with a portal directly to Coldharbour. The assault force is mobilized, and you lead the army into Coldharbour.

Trivia

 * This is the first quest in the Coldharbour storyline.