The Four Horsemen

Overview
Summary:
 * 1) Destroy the four horsemen of Dhuum

Obtained from:
 * Reaper of the Chaos Planes in The Underworld

Requirements:
 * Clear the Chamber
 * Free the Reaper of the Chaos Planes by killing the Terrorweb Dryders around Grenth's statue in the Chaos Planes. They only appear when you have activated the quest Restore the monuments of Grenth.

Reward:
 * 10,000 XP

Dialogue:
 * "Before the time of Grenth, when death was ruled by a cruel and unjust god, there stood a tower and a throne on this very plain. But Grenth rose up and destroyed the one called Dhuum and shattered down his tower, leaving only these storms of chaos as a reminder of the power that once held dominion here. My vigil over these lands was broken for a time, and I can feel a resurgence of the old taint. Four horsemen approach, riders of Dhuum. Destroy them before they can reclaim this place and throw the underworld into chaos!"

Followup:
 * None.

Walkthrough

 * Four separate groups spawn across the Chaos Plains and make their way toward the Reaper of the Chaos Plains in two different stages. Each group consists of one Banished Dream Rider boss (Ghozer Dhuum, Kazhad Dhuum, Marduk Dhuum and Thul Za Dhuum) and three Terrorweb Dryders. During the first stage of the assault, the four groups proceed from their spawn point and merge into two larger groups about one radar diameter away from the Reaper. In the second stage of the assault, the two groups will perform a pincer attack on the Reaper, effectively forcing your party to fight the battle on two fronts.


 * If the enemies makes it to the Reaper, it is very likely that they will attack him immediately. For this reason it is not wise to stay there and try to take all the enemies at once and your party will be kicked out of the Underworld if the Reaper dies. You could try to either spilt your team up completely or to position the weak casters near the Reaper and send your tanks out to block the incoming foes. As they are mostly Elementalists with heavy fire spike damage your tanks should be prepared to face them either with Protective Spirit or some elemental armor like Elemental Resistance or Mantra of Flame.

External Resources

 * The above walkthrough is from this thread on Guildwarsguru.com.