LoreOneExalted/TheTruthAboutShadowlands
Canonically, shadowlands occur when Creation and the Underworld come close enough to touch. Those in Creation hate them, because it brings the taint of death to the land. They fear shadowlands as well, because they act as pathways for invaders from the Underworld to reach them.
What few in Creation seem to appreciate, however, is that shadowlands work both ways. From the point of view of those in the Underworld, shadowlands bring the taint of Creation to them, corrupting the perfection of the Underworld with hideous life energy. An army from Creation intent on conquering the Underworld could have no better ally than a shadowland, providing a route to invasion.
Shadowland Motivations
In this alternative, the above facts color the perceptions of those in the Underworld, ultimately changing their motivations radically. Much of the Underworld aristocracy, particularly the "old school" (i.e. the Dual Monarchy and other pre-deathlord rulers), considers shadowlands to be, at best, a pollution of their realm and, at worst, an eminent threat to their existence. Opinions differ on what would happen if powerful invaders from Creation began to fill the underworld with life energy (for example, with solar circle sorcery), but few wish to find out. When they detect shadowlands, this faction tries its best to keep the area abandoned, so that life energy on the Creation side will eventually grow back into the space and close the connection. With the patience of the dead, they can wait a long time for this to happen, but without active death energy on the Underworld side (or additional death on the Creation side), the result is inevitable.
This faction includes a number of deathlords, but they resist shadowlands for very different reasons. Some are legitimately worried about invasion, particularly from the returning solars, and can't quite fathom why Creation has never even attempted an invasion of any kind. Others are honestly interested in protecting Creation from the dead (see LoreOneExalted/DeathlordHeroes for one explanation of why this might be).
Other deathlords see the shadowlands as an opportunity. While most who think they know anything about the deathlords assume that their intent is to spread shadowlands over the world in order to destroy Creation, the truth is that many of them want to spread shadowlands over the world in order to heal the Underworld. Their reasons for attempting this, however, vary greatly. Some think that healing the Underworld will resurrect their Neverborn masters. Others think that uniting Creation and the Underworld will create a new realm that transcends the limits of both, where the least of its wonders will be that no one ever dies. Some feel that infusing the life energy of Creation into the Underworld will result in the Neverborn being unmade or, at least, jettisoned into Elswhere and, thus, freeing the deathlords and Creation from their clutches forever, at which point Creation and the Underworld could be separated again, into a more pure cycle of life and death. Still others believe that overlapping the realms in this way is the first step in elevating mortal creatures (and the exalts based on them) to a new level of consciousness, with minds that can span infinite realms at once.
Naturally, some deathlords don't believe in the idea of shadowlands healing the Underworld at all, and really do seek to destroy Creation. Whatever their opinion, however, each deathlord has a different idea of the utility of shadowlands and this, more than anything else, is why they fight amongst each other.
