The Chaos Planes, abbreviated as "planes" and sometimes spelled "plains", is the name of an area in The Underworld.
The Four Horsemen
When this quest is taken from the Planes Reaper, the Four Horsemen spawn across planes: Madruk Dhuum in the south-west, Thul Za Dhuum south-west of the centre, Ghozer Dhuum north-east of the centre, and Khazad Dhuum in the north-east. Each horseman spawns with 2 yellow dryders, although the pairs of dryders are their own groups, and will aggro separately to the horsemen. After a moment, these enemies will all start patrolling towards the Reaper, but will stop to fight anyone they encounter. 30 seconds after the quest is taken, a white dryder spawns at each of the horsemen spawn points. After another 30 seconds, a skele will spawn at each point. All of these foes will also run towards the Reaper.
The quest is completed when all 4 of the horsemen are killed.
Planes is entered from the north through Mountains, and has exits to Pits and Pools in the north-east and west, respectively. Most of Planes is a wide open area, with a few impassable chasms. The Reaper monument is in the south.
Apart from the few wandering and patrolling skeles, all of the enemies here are ghosts: Wailing Lords, Banished Dream Riders, and pop-up Mindblade Spectres. All of the ghosts are stationary at first, but each pair of dream riders, together with a wailing, is connected to 3 mindblade pop-ups, and when these are popped, all 6 enemies will begin patrolling as a group. Single dream riders (with 2 wailings nearby) are a different matter. They will not pull far, and when killed, 3 mindblades will spawn at a fixed nearby location. Killing all 3 of these will spawn 6 more mindblades at a different location, and killing these 6 will spawn 9 more. Killing the 9 will not cause any more spawns.