Life cycle:
It is unknown how the Abyss creates these creatures, but those who venture into the Abyss plane and survive will inevitably turn into them. The Abyss comes to permeate their entire being, moulding the creatures affected to become a resident of the Abyss. The Abyss is the darkest plane in existence, you would need a lantern on at all times to be able to see anything. But this is the first of any problems the Abyss transformation addresses.

The Negative energy of the Abyss plane permeates the eyes, and the eyes effectively reverse, the brightness that is perceived. Darkness becomes clear as day when the affected gain the first sign of change, Abyss Vision.

Many people who enter the Abyss plane will find themselves the unwitting prey of one of many Abyssal Beasts that roam aimlessly, raging uncontrollably. For in the Abyss, the only source of food or water lies in the people contained within. There is nothing else but creatures and negative energy, so some who are strong enough turn to the Abyssal beasts for nourishment. Eventually however, the Abyss will change you until you no longer need sustenance to survive. Survivors would also note that they seemed to have stopped ageing.

However, it is after this period that the change truly begins. Those that survive for this long, will eventually find themselves corrupted by the Abyss and turned into the Abyssal beasts which were once their prey. They start to rage uncontrollably and will find themselves losing their sense of self. The appearance of the beast is usually a large hound but sometimes those with strange blood may find themselves to be something else entirely. This typically occurs when the survivor has been in the Abyss for at least 100 years.

After a further 200 years, the beast metamorphosis's back into a humanoid form, a fully fledged Abyss walker. They gain clipped Abyss walker wings and the ability to manipulate negative energy, manifesting as a unique ability for them to use at will. Eventually, as the new Abyss walker learns how to manipulate negative energy, they will be able to craft weapons and armour out of negative energy, and their wings eventually become full and whole.

Sometimes, an Abyss walker never progresses from the beast stage, but instead regains their will and intelligence. These Abyssal beasts usually mutate into more powerful forms and gain the ability to travel to other dimensions at will. However, time is incredibly distorted in the Abyss, and when anyone enters or exits the Abyss they could be going forward or backward in time. Only the Abyssal beasts now how to manipulate their entry and exit, whereas the Abyss walkers who gain this ability can only enter and exit their current time every 3 years. If they try any other time they will be sent back and forth through time.

Actual lore:
Most Abyss walkers came from other planes, either accidentally, willingly or forcibly dragged by the rare intelligent Abyssal beast. However, there were many native Abyss walkers and they took it upon themselves to teach and train other Abyss walkers. Eventually they formed distinct clans and cultures and learned to live in the Abyss.

It should be noted that native Abyss walkers are not exclusively those who were born in the Abyss. Rather, the native Abyss walkers mainly consist of individuals who have been in the Abyss since an extremely young age and as such most native Abyss walkers look to mainly be in their teenage years for their respective races.

Although some Abyss walkers had the ability to walk between dimensions, most chose not to as they believed they would be alienated by their homelands. Those that did would sometimes find their homes had been gone for millennia and returned with heavy hearts to the Abyss plane. One way or another, almost all Abyss walkers stayed in the Abyss realm to live their infinite lives in peace.

That is until the appearance of Jund the World Destroyer.

Jund the World Destroyer was a mysterious figure who was shrouded in mystery. His destructive powers were unmatched, and he could wipe out cities with a single spell. No one knows why, but he sought to wipe out the Abyss walkers from the Abyss plane. Only one person is recorded as having survived to tell others, but regardless, the population of the Abyss plane almost ceased to exist. However, there were still a few Abyssal beasts who sought others to drag down to the Abyss plane.

All this stuff happened in my other pathfinder by the way.