The Four Horsemen

Overview
Summary:
 * 1) Destroy Madruk Dhuum
 * 2) Destroy Thul Za Dhuum
 * 3) Destroy Ghozer Dhuum
 * 4) Destroy Kazhad Dhuum
 * 5) Return to Reaper of the Chaos Planes for your reward.

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 will spawn along the northern edge of the Chaos Plains and make their way toward the Reaper of the Chaos Plains. Each group will consist of one horseman (Ghozer Dhuum, Kazhad Dhuum, Madruk Dhuum and Thul Za Dhuum) and three Terrorweb Dryders.

The two groups of Ghozer and Khazad will merge about a full radar circle to the east of the reaper, and the groups of Madruk and Thul Za will do the same about the same distance to the west. The combined two mobs will approach the reaper shortly afterwards from the east and west simultaneously.

Fighting at the reaper may not be the safest approach as the combination of the Dryder damage and the horsemen's mesmer skills will make protecting and healing the reaper a difficult task. The horsemen have enchantment removal as well as Power Block which can shut down a caster.

One strategy may be to split the party into two groups and try to hold back both sides, another would be to leave a heavily fortified tank on one side ahead of the reaper and heal that tank with distance healing (like Heal Party and Light of Deliverance) while the rest of the party takes out the other side then rush to his aid. It's important to note that spell-coverage, like Spell Breaker goes a long way in denying these hordes the ability to do damage.

Trivia

 * The Four Horsemen may be inspired by The Four Horsemen of the Apocalypse.
 * Thul Za Dhuum is likely in homage to Robert E. Howard's Thulsa Doom of Delcardes' Cat.
 * Ghozer Dhuum is likely in homage to Gozer the Gozerian, the supernatural foe from the movie Ghostbusters.