Assault on the Stronghold

Objectives

 * Roan Fierceheart in Dalada Uplands will prepare you for the assault. (disappears from Quest Log after speaking to Roan)
 * Invade the hierophant's stronghold and rescue the Ebon Vanguard.
 * Defeat the Charr guards and gain control of the siege devourers.
 * Target archer groups with siege devourers to cover the armored saurus while it breaks down the front gates.
 * Free the Ebon Vanguard held captive within the fortress.
 * Defeat Hierophant Burntsoul and all hostile forces.

Obtained from
 * Roan Fierceheart in Dalada Uplands (automatic on completion of What Must Be Done)
 * Scrying Pool in Hall of Monuments (after first completion, select "I seek guidance" option)

Requirements
 * What Must Be Done

Walkthrough
Start by exiting Doomlore Shrine to the south, turn right, and talk to Bonwor Fierceblade. Accept the quest dialogue and you will be teleported to Sacnoth Valley where the mission will begin after a lengthy cinematic.

Run up to your Charr allies and they will assist you with taking the Siege Devourer on the right. They will then hold the siege devourer while you and your party take the one on the left. You will receive a morale boost and then the second phase of the mission begins.

The siege devourers will start to pound the Charr Seekers on the wall. Charr warriors will constantly spawn in threes in the two glowing circles next to the wall, and charge toward the siege devourers and try to kill them, thereby stopping their barrage. In the meantime, an Armored Saurus will spawn behind you and run toward the gate. It will attack the gate while the Charr seekers on the walls try to kill it. The saurus will die before it reaches the wall if all seekers are alive. It will likely still die if a few seekers are alive, but a new saurus will spawn to take its place. The seekers will constantly respawn on the wall until the gate is breached.

Defending the siege devourers is not difficult if you kill the spawning Charr quickly. If you let them go, more will spawn and you will soon be overwhelmed. The player's party can defend the left siege devourer while the Charr allies protect the right siege devourer. However the siege devourers do not kill enough Charr fast enough by themselves to protect the saurus, so you cannot (?) simply stand back and protect the left siege devourer and wait for the saurus to breach the gate.

Therefore, once the left siege devourer has taken out a few seekers on the left side of the wall, move to the glowing circle where the Charr warriors spawn. Using ranged attacks, take out the seekers as they spawn on the wall. Start from the far left and move inward. You will also need to take out the Charr warriors as they spawn on the ground. They will attack you before they rush the siege devourer, so the siege devourer will be unmolested so long as the warriors spawn in someone's aggro circle. Leave the right siege devourer to your allies. If you can keep the seekers thinned out and Charr warriors from attacking your siege devourer, eventually an armored saurus will break down the gate, and the third phase of the mission begins.

Run through the broken gate and to the right. Clean up any remaining Charr that might attack you from behind, then approach the mission marker in front of the stairs. Once you reach it, a dialogue will begin. Do not climb the stairs. Instead, back away and let the spawning Destroyers wipe out the Charr on the plateau. With luck, they will mostly wipe each other out. Once there is nothing left but Destroyers or Charr, move in to finish the remnants off. Once all enemies are dead, the mission ends. It seems random which 'species' is left standing at the end, Charr or Destroyer.

Bosses

 * Charr
 * 29 Hierophant Burntsoul (Ether Prodigy)

Dialogues
Briefing from the Scrying Pool.