Doomguard
Ever hear of entropy, berk? Take a look around: Everything's going down the tubes, falling apart, stopping. People die, rocks erode, stars fade, planes melt away. That's entropy, the fate of the multiverse. A lot of folks think that's a terrible thing, but not the Doomguard. They're pretty sure nothing lasts forever, not even the planes. It's the way things are supposed to be, they guess, the goal of everything. Sooner or later, the last bits of the multiverse'll decay, and then there'll be nothing left - think of it as existence's ultimate release from toil and pain.
Now, the sods who want to fix things - stop the decay and put everything back together - they've got it all wrong. They're fighting the natural order of the multiverse, trying to do something unnatural. That ain't right.
So look, the Doomguard's here to see that the multiverse gets its way. Things are supposed to crumble, and it's the Doomguard's job to keep the meddlers from messing it up too much. What right do mortals have to deny the natural existence of things? And somebody's got to watch the proxies and the powers, to make sure they don't meddle with the process. Can't have the powers restoring things or ending them too fast, you know.
Don't get this faction wrong. It's not like somebody builds a house and they tear it down. That building's part of the whole decay: The stonecutter chips the rock, the logger cuts the tree, and later the termites chew the beams until the whole case comes down on its own. There's a long view to this. The sod who can't see the grand scheme'll go barmy trying to toar down everything that gets built. So, everything's got a part in this. The primes slowly eat away their worlds, and planars do the same. Look at petitioners - entropy reaches perfection when they fade away. It'll all happen in time.
Allies and Enemies
Both the Bleak Cabal and the Dustmen find the entropic visions of the Doomguard well suited to their own philosophies, although the Cabal sneers at the idea that entropy is the "goal" of the multiverse. The Godsmen agree with the idea that the multiverse is fated to end, but they can't accept the idea that destruction is the purpose itself. The Fraternity of Order and the Harmonium reject the Doomguard's philosophies wholesale.
Powers
As agents of entropy, members of the Doomguard find a natural affinity for powers who bring destruction to the multiverse, whether on a grand or local scale. Thus, they count Talos the Destroyer (of Toril) as one of their patrons - with a name like that, how could he be wrong? The faction also worships Shekinester's aspect as the crone-faced Weaver (though the other two aspects, which suggest building, are right out). Vayu, the randomly destructive power of wind in the Vedic pantheon, is also high on the list.
However, the god most highly revered by factioneers is Siva. After all, it's his function to bring and end to the multiverse. Sure, Siva occasionally acts to preserve, but only to make the eventual destruction that much more complete. And yes, Siva might be aiming toward a greater whole, but the destruction comes first. That counts for a lot with the Doomguard.