The D'Karr and the Karr

Full Description
Long ago, in an event that is barely exists in any human knowledge, humanity split. The recorded reasons are hazy at best, but it is guessed that two major leaders of early humanity differed in view about how the planes they had found were to be colonized. Humanity as we know it traveled upward, and the D'Karr traveled downward. The D'Karr begun their civilization on a much colder and darker plane than the sun baked surface, for they had traveled past the dwarves and deeper into the Underdark. The place they reached was completely devoid of light and had a strange, even eerie feel that seemed omnipresent. Settling down and hunting in this new environment, the D'Karr used many light sources, but eventually evolved an entirely different sense at the expense of their eyesight. Those that have been encountered outside their home are said to be able to 'sense' their surroundings with a kind of telekinetic field that can determine living from inanimate. With their new senses came new methods of stealth, where some D'Karr can suppress their electrical field and slow their heartbeat to the point of being undetectable to other D'Karr. In addition to the changes in their senses, the D'Karr lost the pigmentation in their skin and grew longer fingers, but otherwise remained humanoid.
As the human civilizations flourished, so too did the D'Karr civilizations, but instead of studying magic like their above-world cousins, they developed their internal psionic ability. The D'Karr government was built around religious knowledge, and a council of town representitives and religious leaders acts as lawmakers and exectutive leaders. There is a seperate court system that relies on mind reading to accurately give verdicts. As the D'Karr grew, they came into contact with the dwarves, the deep elves, and the goblins, with whom they traded, until the goblins decided that these strange humans were a threat and started an attempted genocide. Uniting swiftly and decisively, the D'Karr allied with the dwarves, beating the goblins into submission in what has to have been a record time. Emboldened and enriched by the victory, the D'Karr decided to try exploring the ground under them to see if some other world lay beneath their feet.
The first major expedition was celebrated across the shadowy plane they lived on, but after a few days of exploration, the party was never heard from again. Understandably concerned, the D'Karr sent an elite team of warriors to face whatever threat lay below the surface. The party was kept under continuous surveillance, but at the point where the previous party disappeared, the psionic sensors were mysteriously deactivated and no communication was received.
This time, the D'Karr sent their entire army to the mouth of the cave that had taken two of their teams and ordered the army to make camp and prepare for an enormous assault. Just before the attack was ordered, one of the members of the elite squad of warriors came stumbling out of the cave, terribly mutated and completely insane, muttering strange phrases that concerned future events. The attack was stalled as the most powerful wielders of psionics came together to determine the source of this terrible power. Inch by inch, they uncovered that both parties discovered a roiling chaos of destructive energy below the cave from which every kind of horror imaginable spawned from. The explorers had been consumed by some drifting entity that seemed only intent on mutilating other creatures.
While the D'Karr where relieved that they were not fighting an organized enemy, they decided that this entrance and every other entrance from the chaos to their world must be sealed off, guessing that they could not destroy the plane below them itself. After identifying locations that could be used to go between the two planes, they constructed enormous seals that deadened all travel near them, and built fortresses to protect the seals. By the end, their plane was dotted with fortresses protecting the inhabitants from the chaos beneath.
But the chaos had a much more subtle effect than expected, and as more resources where used trying to understand the nature of the enemy, more citizens of the D'Karr decided that they should try to harness the energy of the plane. Whether or not the experiments were started with good intentions is unknown, but many prominent figures in D'Karr society began experimenting with connections into the realm beneath them. After a few of these experiments got out of hand, something changed the experimenters, possibly the thing that had killed the D'Karr explorers, destroying their sanity and convincing them to spread the chaos they felt into the world around them.
The wars that followed were some of the bloodiest fought by any race in existence, with both sides developing more and more sophisticated tactics of annihilating the opposing forces. In the wake of a huge battle for one of the main seals, the normal D'Karr began to gain an advantage and eventually defeated the corrupted D'Karr. As the wars drew to a close, the remaining corrupted D'Karr were rounded up and placed into prison camps. But as they stayed there, the D'Karr leaders could not decide what to do with these unproductive, dangerous, madmen. Then they hit upon a solution in which they erased every memory from their prisoners, sending them out of their realm with an ancient mark of shame tattooed on their chests to warn the other races of the crimes committed by these creatures. The process of putting the mark on the corrupted D'Karr permanently removed their hair, and made them more feral, with small fangs and claws. This new 'race' was referred to as the Karr.
With the Karr confined to the Underdark above them, without any aid from the other races, the D'Karr relaxed and began to rebuild. Everything went well for a few years, as the Karr warred between themselves, and the D'Karr re-sealed the chaotic plane off from their realm. But things changed when a goblin alterted by the same chaos that the D'Karr had fought off years before learned of the Karr. This goblin slowly rose to power in the Goblin Empire that was still shattered from its war with the D'Karr. In a surprise coup, he took control of the government, and as his first action as Goblin Emperor, offered aid to the Karr.
Now, to offer aid to a creature with the mark of shame placed on the Karr was a violation of the most ancient of guidelines for civilizations, but no one in the Goblin Empire had the interest or power to oppose the new Emperor. So the Karr where united as a force once again, this time equipped with the mechanical genius of the goblins. Their first target was the D'Karr home plane. But the D'Karr had seen this coming in a set of prophecies, and instead of fighting the army that was rapidly descending upon them, they sent a team of elite assassins that were able erase their presence from the minds of their opponents to kill the generals of the army. Expecting a straight war, the goblins and the Karr were caught completely by surprise. The Culling, as it was called, was so successful, that the assassins decided to attack the Goblin Emperor himself. The Emperor was easily found and dispatched without his army to protect him, and the goblins and the Karr fell into disarray once again.
What saved the goblins and the Karr was the fact that they had both violated the ancient law concerning the mark of shame, and so they re-banded together for survival. While the D'Karr tried to warn the other Underdark races about the new danger facing them, the goblins and the Karr attacked the dwarves with a purpose, feeling that they were not yet prepared enough to attack the D'Karr again.
The Karr and the goblins were successful initially, but after they started to get closer to really hurting the dwarven empire, its defenses suddenly tripled with aid from the D'Karr. Stuck in a deadlock, with the deep elves staying neutral, the Karr and goblin forces moved to the next unsuspecting foe, the humans. The Karr were amazed by the size of the human world and the sun that burned Karr flesh, but nonetheless started fighting. Simultaneously, the D'Karr found that the corrupted D'Karr weren't gone from their realm when two fortresses were seized, and the seals they contained were broken.

Additional Information
This new attack on the upper world of humanity is the first major contact that humans have had with their estranged cousins since their separation, and no one knows how they will react. The D'Karr have considered sending envoys to the humans, but in light of the present turmoil they are involved in, it is unlikely that they will. Another impediment to a negotiation between humans and D'Karr is that for the most part, the D'Karr have forgotten about the humans as much as the humans have forgotten about the D'Karr, and the only reason that they have any idea humans still exist is from dwarven reports of areas that have been attacked by the Karr and goblin forces. One major step in defeating the Karr and the Goblins may be convincing the other races, including the deep elves, that the Karr/Goblin force represents a common threat to all.

The PC's in your campaign could be charged with fighting the Karr or going down past the underground to find the source of these 'monsters.' They should definitely run into both subraces along the way. Either way, make the difference between the Karr and the D'Karr be like night and day. The PC's will be all up in arms, and then greeted by the high priest, who invites them to a meeting of the ruling council. Or they are fighting bloodthirsty Karr, and a force of strangely similar creatures bust in and massacre the remaining Karr. Also, any rouge in the party will get a rude suprise when he can't sneak anymore, the Karr and the D'Karr will be able to see him no problemo.