The Origin 

Rapdread race can track their origin back to two original ancestors: Kwuintla and Gorxog.
Kwuintla was a female whose parents were a dragon and a lizardfolk. She was a powerful shaman of her people, but when her tribe lost a war against an orc tribe, she got lost and was taken away into the dark world by her mate.
Gorxog is a demon. In the Abyss he made a lot of enemies and was fleeing when he stumbled upon a gateway leading into the Voidlands. After some time in the material plane, he and his lover (Kwuintla) travel through the portal and start a new life far away from anyone and anything.
All Rapdreads can trace their lineage to two originators of their race, though current Rapdreads barely resemble their ancestors. Thousands of generations and adaptation to unforgiving life at the furthest edge of the Voidlands can and will change anything and everything.
When it comes to the life of Rapdreads now, they fall into one of two groups: wild ones following the will of their ancestral mother and somewhat civilized ones.

At the Edge of the Voidlands 

There exists a long lost realm. It is the last remaining remnant of the first civilization within the cosmos, built by ancient elementals. However, now it is just a crumbling realm balancing on the edge of oblivion. This place is dark and cruel, and the further away from the center of this realm you move - the more cruel it becomes.
It is hard to exactly tell where the Voidlands end. At the furthest corner of this realm everything is shrouded with Null Fog - a mist of oblivious slowly swallowing and erasing everything it swallows. Wherever this fog emerges, that place could be named as the true border between existence and non-existence, and this is the place where the Rapdread race emerged.
Fleeting and fading, the unforgiving life at the edge of the Voidlands is beyond harsh. This is where ancestors of the Rapdreads made their life, had their children and unleashed their scourge upon the lost realm. With the only direction of movement being away from oblivion, Rapdreads have no other higher purpose than to invade, fight, conquer and dominate. For them, life exists as a short fleeting experience in which the only thing they can do is to fight. Well... at least to those who prefer to live this way. Some do indeed desire a more meaningful life, but there are much more of those who do not.

Mother of the Legion

Kwuintla was a powerful caster of magic. She was able to manipulate elemental forces and she was even able to invoke the breath weapon of her father, because her father was a dragon. Her blood runs deep within all of Rapdreads, though her magical capabilities are not inherited by most of her descendants. However, if any of her descendants do show any talent towards magical arts, then it is quite normal for them to seek guidance from Kwuintla.
Kwuintla lived for about 184 years and eventually died from old age. However, she did not remain dead for long. Her soul was called back by her husband, and she resurrected as an undead, something quite similar to a lich. While alive, she was somewhat affectionate towards her descendants and her partner, however, when she became an undead - she became cold, cruel, driven by a desire to take over the entire realm of the Voidlands, and then to take over the entire universe. For this goal she guides her descendants to fight for her, to die for her, and then to be animated as undead for her. At the edge of the Null Fog - millions of undead soldiers are waiting for her command, waiting to be unleashed as scourge upon every other creature of the universe.
Long years of isolation, undead nature, or maybe simply because life in the Voidlands is absolutely maddening - Kwuintla is now nothing but a cruel overlord ruling over her descendants. Some could say that the current appearance and adaptations of the Rapdread race are because of Kwuintla's influence, but very few scholars would be brave enough to test out this theory...

Life of a Rapdread

Typically, a pregnant Rapdread female lays between two and four eggs. Each egg is then hidden in a hole, between tree roots and some other place where they could mature and adapt to their surroundings. If the mother has time, then she typically covers the eggs with branches, leaves, animal body parts and other materials the egg could siphon for sustenance, but that happens rarely.
After being laid, the egg will remain mostly inert for around 20-30 days, though it has an ability to siphon nutrients from its environment to help the young Rapdread to develop faster and to be born stronger. When time comes and youngling is ready to hatch, it happens quickly, and the young Rapdread emerges ready for battle. From the first minutes of its life, the youngling is guided by a very strong impulse to hunt and fight, to tear apart anything they see. Therefore, female Rapdreads never lay eggs around their settlements, but instead do so around the location where foes are abundant. Either due to their nature or because of their ancestor's wishes, Rapdread young are used as living weapons...
If a young Rapdread manages to survive for the first few weeks of their life, then most of them will eventually find their way into any Rapdread tribe where they will be immediately welcomed in as full fledged adults. As adults, they are responsible for hunting for food, going on raids against other creatures, and... reproduction... The life of a typical Rapdread is short, filled with gruesome battles, and only the strongest survive long enough to maybe one day meet their own grandchildren, though, even then the Rapdread society does not recognize blood connections not only between distant siblings, but even between parents and children...
To most outside observers, Rapdreads may look like a savage race, more monsters than someone you could have a conversation with. However, that is only a half of the story.

Gorekan's Settlement

Rapdreads have various settlements between the edge of the Voidlands and the central city. Most of these settlements are very similar, filled with hunters and gatherers, without anyone who has the luxury to do anything else. But there is one settlement different from all others - Gorekan's Settlement. It was established by the second son of Kwuintla and Gorxog, and to this day all inhabitants of this settlement believe in his worldview. Gorekan, the second son, believed that Rapdreads are not meant to live as savages and that much more could be achieved through cooperation with other sentient races dwelling within the Voidlands.
Rapdreads of this settlement partake in trade, managed to build some diplomatic relationships with other sentient races, they even have artisans and a monetary system. While still living in somewhat primitive conditions, they have record keepers, a working government of sorts, and this settlement is slowly developing into an actual metropolis, though it is almost entirely built in trees out of wood, grass and dirt.
Whenever an outsider meets a friendly Rapdread, then they could be certain that the individual in question was raised in Gorekan's Settlement, but even then people should be vary, because savagery runs deep within this race.
Not many individuals of other races have been in Gorekan's Settlement. It is only open to the Rapdread race, and even then some are rejected. To maintain a civil and somewhat peaceful way of life, inhabitants of this settlement enforce strict control over its borders. To enter the settlement, you must have a referral and you must bring some gifts for the locals. Then, you are informed about the rules of the settlement: what you can do, what you cannot do, and what to expect while staying in the settlement.
From time to time a scholar or two manage to find their way into this settlement. They always request to copy the records of the city, to meet Rapdread scholars and to share the knowledge, to share the experience of living in such an unforgiving place. Some scholars who manage to return to the central city or even venture out of the Voidlands do describe Gorekan's Settlement as the last bastion of sentient life before the absolute oblivion. In writings of scholars, the Rapdread race is often described as "strange fiends" who live either as cunning savages or live in barely civilized settlement where they pretend to be friendly while menacing their fangs at anyone who dares to meet them in person...

Zaradina and the Failed Plot Against the Abyss

Few hundred years ago Rapdreads made themselves known by trying to assault a layer of the abyss. A sole tribe was led by a female shaman named Zaradina. She felt a calling to embrace her demonic lineage and she was filled with the desire to return to the realm of her distant forefather.
One day Zaradina ordered her tribe to march through the jungle of the Voidlands, towards the sealed away portal into the Abyss. She and her loyal shamans unsealed the portal and allowed both sides to pass through. However chaotic the Abyss might be, barely anyone would have expected an assault by never before seen creatures within a deep layer of the Abyss. Against all odds, Zaradina and her tribesmen were very successful - demons were being taken down left and right, demonic settlements were overrun and turned into outposts for the invaders. And the longer the invasion lasted - the stronger Rapdreads became. Embracing their demonic blood, feeding on the left and fallen demons, Zaradina saw her people grow in power and she loved it. For a brief moment in time she believed that her plans will succeed and that she alone will become the ruler of the Abyss, but this dream did not last long as a demon lord of nearby layer descended upon Rapdreads, washed them away with flames, and ripped out the souls of those who managed to survive the descent. What happened to Zaradina is not known. She may have perished with her people... or maybe she had enough cunning to convince the demon lord to take her up as an underling. Only time will tell what is the after match of the failed plot to conquer the Abyss. 
The whole ordeal made demons aware about the existence of strange creatures, but not to the origin of where they came from. So, given an opportunity, demons might try to contact Rapdreads for whatever benefit they may seek. The connection between Voidlands and the Abyss may only strengthen over time, and it could spell a disaster to the material plane...

Crisis Within the Voidlands

Barely anyone outside of the Voidlands has ever seen a Rapdread. Well... until quite recently. Few decades ago something started to change. Influence of the Voidlands started to grow. Connection between this lost realm and the material plane started growing, various people started finding their way into the Voidlands, and... of course, various creatures started finding their way out of the Voidlands.
Within the Gorekan's Settlement a group of Rapdreads suggested a plan to venture out, to find a portal into the material plane and establish their own outpost there. Either for trade, for diplomatic relations or to simply explore unknown worlds, any justification could have been made to explain why the most curious of their kind would even attempt to travel into unknown worlds. While Voidlands entered into a sort of crisis, some Rapdreads saw that as nothing but an opportunity. Where this opportunity leads to only time will tell, however, into whatever world they might travel to - everyone can be certain that Rapdreads will adapt to it, and then thrive in it.