You round the bend in the underground tunnel and a cavern opens ahead of you. The faint rays from your flickering torch disappear into the yawning gloom ahead. You make your way down to the floor, miniature rock-slides marking your progress. When you reach the bottom, you brush the heavy dust from your clothes and turn to resume your trek. You freeze as the torchlight flickers over something... strange. After a moment's hesitation you move cautiously forward to gain a better view, halting several yards from the oddity.
A large irregular sphere, pale yellow-green and roughly six feet in diameter, hangs suspended before you, perched atop numerous slender filaments that stretch down to pierce the ground beneath. The surface glistens with slime and large pores with swollen rims, each a hands-breadth across, dot the lumpy skin like a multitude of festering wounds. At first it seems odd that such ponderous bulk rests on such seemingly fragile supports, but the longer you gaze at it, the more you feel the central mass actually supports and anchors the filaments as they stretch down into the earth. You become convinced when your scrutiny reveals that while the center mass is motionless, the filaments are continuously undulating in a nigh-imperceptible writhe.
As your eyes pass over the area where the thing's tendrils enter the stone, a small glint catches your attention. Several small objects gleam in the torchlight, and they seem to be spilling out of a half-buried pouch. Coin! -- a matching gleam sparkles in your eye as greed compels you forward. Not wanting to move any closer than necessary, you halt several feet away and stretch out your sword, hoping to catch the pouch with the tip of your blade. Suddenly a long drops and ricochets off your weapon. With a start you leap back, eyes darting up. From every pore the fungus oozes a mix of yellow ichor and bone, all falling into a slimy pile on the ground. Ribs, shoulder blades, a spine, and finally a skull. An inhuman, fang-toothed skull. A moment after the skull hits the pile, the bones begin to rise and join together. Seconds later a full skeleton stands before you, yellow ichor flowing over and around each bone in narrow rivulets. Jaws open in a silent roar and the nightmare steps towards you...
[Before we go any further, allow me to note that the Necromould is not a monster per se, but can be thought of as a flavorful explanation for the presence of undead, both in the world and in a particular location. The Necromould can also serve as the existential threat against which the PCs are striving]
It is said that long ago, an elder god from a distant dimension was cast down and murdered, betrayed by his most trusted servants. With his last strength, the old god uttered a curse of vengeance that consumed his own essence and gave rise to the Necromould. The Necromould may best be classified as an unliving disease, an unholy cancer that spreads decay and death wherever it propagates. The Necromould quickly consumed all life in its home dimension, and now strives to spread to other planes to feed anew.
While the vast majority of the Necromould's physical bulk resides in its native realm, it is constantly sending out astral feeding tubes that probe other realities. When one of these feelers happens to alight on a living creature at the time of its death, the life-force acts as a conduit through which the Necromould can deposit a spore in creature's body. Part of the creature's soul is drawn back through the astral feeder to the main body of the Necromould, to languish in uncomprehending torment. A sliver, however, remains behind to nourish the pod. Each necropod exists to consume life, and catches its prey by reanimating the dead. These undead minions may be the remains of the creature that first served as the gateway of the necropod, or may merely be the bones of some other dead creature. Indeed, the necropod extrudes tendrils which slowly phase into the ground, always searching for a stronger corpse to reanimate. On finding such a prize, the necropod magically draws it up through the earth and into itself, ready to act when needed. When a living creature approaches within a certain distance to a necropod, roughly equivalent to the diameter of the pod itself, the horrible mass begins expel the physical components of the corpse it has animated. In a matter of seconds, the skeleton, zombie etc is made whole, held together by faintly glowing necroslime. The undead minion remains tethered to the necropod via an astral feeder, but is otherwise able to move freely in pursuit of living targets. The minion will attack all living targets, and if successful in making the kills, will bring the corpses back to the necropod. The newly fed necropod may grow, and may "elect" to animate the fresh kill.
Necropods vary greatly in their own size and in the size and number of creatures they control, based on how many lifeforms have fallen prey to them. Mechanically speaking, each necropod has a diameter proportionate to the number of hit-dice worth of undead it commands. The ratio is three feet of diameter for every 1 hit-die. Thus a 15-foot diameter necropod might contain five human skeleton warriors at one hit-die apiece, or a single 5 hit-dice owlbear skeleton. Or it might contain a 2 hit-dice ghoul, and a 3 hit-dice shark skeleton. Of course, the shark skeleton won't be terribly effective in combat, but the necropod is effectively non-intelligent. It cannot choose it's servants selectively, it merely seeks out the echos of the most powerful form. Necropods of sufficient size may even contain intelligent undead, which are capable of independent thought, but are nevertheless compelled to serve their master's hunger. There is no upper limit to the number of monster hit-dice a necropod may contain.
Even if a living creature survives an encounter with a necropod's minions, the danger is not yet passed. If the necropod managed to injure the living creature during the fight, then the living creature itself now contains traces of the Necromould. The creature is now sensed by the Necromould, and when the creature dies, there is a 1 in 6 chance that the Necromould injects a spore at the moment of death. It may or may not be within the power of certain clerics to help a victim to avoid this fate. Necropods are mostly immune to physical damage, but their minions have all of the normal undead vulnerabilities. The only known way to destroy a necropod is to sever the astral feeder tube that connects back with the Necromould, and then to burn the now-vulnerable pod with fire.