This is your standard siege-style quest. Speak with Glayvin to get the quest, then talk to Justiciar Naveed (who is standing nearby) to be transported to an instance of Tarnished Haven called "The Justiciar's End". There will be five waves of White Mantle assaulting from the left side, consisting of White Mantle Priests, Savants, Sycophants, Seekers, and Zealots, all level 16-20. Each wave will spawn after the previous one has been defeated. Lazarus the Dire will appear as a non-aggressive NPC after the last wave.