The Bonecloud is a massive, unmoving, and quite hazardous feature of the Astral Plane, composed of literally billions of undead all clustered in a single mass almost three quarters of a mile across.
This phenomenon has its origins on the world of Terras; specifically, an incident known as the War of Lies. During this battle, an ancient lich known as Kolin summoned up a horde of undead, the likes of which haven't been seen on the planes since. This army was so massive that it was said not a single body remained on that world after he was done. A trio of men faced off against this foe: a wizard whose name has since been lost to time, and a pair of priests of light. These three, working together, opened a rift to the Astral, sending the entirety of the army of undead into the Void.
Once on the void, these undead were thankfully for the most part stuck where they were sent, being mostly composed of mindless automata. Soon, however, the force of this many dead in one spot somehow opened a tiny rift to the Ethereal, and through there a pathway to the Negative Energy Plane. This energy allowed for the occasional formation of a mindful undead among the mass, though these are still by far the minority; the occasional ghast or ghoul feeding on the other undead, a small number of wraiths or vampires drifting through the cloud, never drifting far without risking ending their own existence. By and large, the cloud is almost entirely composed of skeletons and zombies (humanoid, animal, and monstrous all).
Traversing around the Bonecloud requires two to six hours, due to the mass of negative energy creating such a strong mental drag. However, travel through the Bonecloud, though dangerous, is possible. Fact is, a number of bloods have created homes within the Bonecloud, using undead-destroying magic to carve out a bubble within which they can dwell.
There are many that are forced to travel into the Bonecloud, for a number of color pools a basher may want access to are located within. Pools near Heart's Faith in Mount Celestia, Rigus on the Outlands, and Alfheim within Ysgard are all located within the cloud's boundaries. Further, some say that a conduit passing through the cloud was purposefully ruptured, forcing any attempting to use it to end up right in the heart of the mindless mob.
- A Guide to the Astral Plane, pp.80-82