Dam word count...
![Mad :x](./images/smilies/madashell.gif)
The creature only requires the heart because it uses the heart to creature more of its own kind. (More about that later).
The creature is physical, so it is solid and fairly tough, but because of its incredible stealth capabilites and lack of psychic detection, it makes them very dangerous and virtually impossible to defeat in melee combat. Its claws inflict 4d6 damage, but its bite does 1d4 x 10 and is capable of slicing through most conventional body armor like a hot knife through butter. It however only uses it bite attack when engaged in actual combat, and uses its powerful bit to rip out entire chunks of flesh and bone from its opponent much like a shark would.
The creature is impervious to normal weapons, but weapons of iron (yes this demonic predator has its origins in the dimension of Krynn) always inflict normal damage. Holy weapons inflict double damage. Cold and fire have no effect though if a fire is large enough it may reveal the outline of the creature. Bright artifical light will hold the creature at bay, but true daylight is lethal and will kill the creature within 1d4 rounds unless it finds secure shelter. Globe of daylight only hold the creature at bay and inflicts 3d6 damage. Magic attacks have no effect and psychic attacks won't work nor should they be tried (another deterant because it is creature from Krynn).
As for how these creatures come about into our plane is that ironically they are invited in. In every large city throughout the world with a cathedral (this is important for some reason), there is a seal of solid gold located deep below the city somewhere. Only a mortal can break the seal (as human greed is all that required to break the seal). No matter how much care is used to remove the 3 foot disk of gold, it will shatter into 3 pieces. This will open a entrance etween our world and the creature's lair in Krynn. Usually those that first break the seal are the first to die; their reward for releasing the creature into our world.
Upon entering our world the creature will immediately begin hunting for victims. It however is precise in it prey, as it seeks the "unwanted and undesirable" amongst the city's populace; homeless, drug addicts, prostitutes, run aways, etc. It attacks, extracts the heart, and returns to is lair to "plant" the heart. Deep in its lair the heart acts as a unholy seed that literally grows a new Shadow Stalker like some demonic fungus. Once the new Stalker reaches maturity in seven days, it ventures forth and begins hunting. Within weeks their can be dozens of these supernatual predators stalking the dark places of man's domain. Once their number reaches just 18 they can disrupt the city's power flow and literally black out an entire city plunging it into blissful darkness. Once their numbers reach 36 or more, they will start killing for pleasure and will render their victims to pieces for the sheer pleasure of it.
The only way to stop this infestation is to stop the original creature. Killing the creature before it "breeeds" however is not in and of itself going to stop future incursions as the portal is still open. The only way to truly stop this from happening again is to enter into the underground and seek out the portal and reseal it using the fractured pieces of the seal. Magic is not required as long as at least three GOOD aligned poeple selflessly reunite the fragments (and give up 3 permanent points PE) and place it back over the opening and sprinkle it with holy water. Another means to seal the void is to have a character of GOOD alignment to actually enter the portal into the creature's lair as an act of self sacrafice. At this point the portal reseals itself again but the character is lost forever, as they become the next Shadow Stalker to replace the one that was slain.
I can provide actual stats if requested.