The Twin Serpent Mountains, abbreviated as "mountains" or "mounts", is the name of an area in The Underworld.
When this quest is taken from the mountains Reaper, the Slayer spawns in the centre of the mountains area, together with numerous Keepers, white dryders, and yellow dryders. All of these begin moving towards the Reaper, but the Slayer is much faster than the dryders, so it will arrive long before they do. All the enemies will stop to fight anything they encounter along the way, however.
The quest is completed as soon as the Slayer dies. None of the other enemies need to be killed, but they will continue walking to the Reaper as long as someone can see them.
Mountains is entered from lab in the north, and there is a passage to planes in the south-east. The Reaper's monument is at the top of the mountain in the far west of the area. At the foot of this mountain are three pathways: one leading to lab, one to planes, and one through the middle of mountains. There is another path around the east of the area, between lab and planes.
As well as Skeletons of Dhuum and Dying Nightmare pop-ups, there are several Charged Blackness patrols, each consisting of 3 enemies. However, most of the enemies here are Obsidian Behemoths. Although they are immobile, and each group has a fixed spawn point, the position of the individual Behemoths is randomized within the area the group spawns. This means that although each group will appear in approximately the same place every run, it's impossible to predict exact where each Behemoth will be. This can result in a group completely body-blocking a narrow pass and the T2 (or whoever is responsible for popping mountains) will sometimes declare that the group at the monument is in a bad formation, or a good one.