Several leagues from the entrance to the hidden valley, one of the multitude of dimensional portals had appeared, its opening heralded by a growing stillness that was soon broken by the crackle of dark energy in the center of a small glade, followed by the sound of tearing fabric as the portal stretched open, much like the pupil of a cats eye. No matter what angle you faced the portal from, it appeared the same way, an eclipse with pointed ends, much like someone had cut the fabric of reality and now held it open. Peering into the portal was pointless, everything was a distorted haze, like looking through a poorly made bottle, no distinction of shape or form, just motion and flashes of light, similar to lightning burst but without any thunder being heard.
It was from this gateway into the physical realms that they came, a vast horde of demons pouring out in every direction with whoops, howls and varied cacophony of demonic calls. While most seemed content to mill about aimlessly, chattering away in excitement at being set loose, others moved with definite purpose, some taking to the air, spreading out with watchful eyes and keen senses. Others took off a quick run, hunting the wildlife in the immediate area, killing skinning and dragging the carcasses back to the majority, saving the cleaning of their kills for later. They were demons, there wouldn't be much left of the prey as it is, why waste anything one of them probably had an appetite for? As this went on, others had come through and begun felling trees, clearing the area and expanding outwards, the wood being used to construct a crude camp, the skins and bones that didn't get eaten, used to decorate things in a simple, brutal fashion.
Those without set tasks, or simply to lazy to work yet, all turned a frightful gaze towards the portal, some of the smaller demons beginning to tremble in abject fear. The reason soon stepped into the realm, the ground beneath its cloven hooves decaying away with each heavy step. Twelve feet tall, dark crimson red in color, a hide and skull girdle about his waist holding his loincloth up, though it only barely covered his dignity, diagonal leather straps crossing from hip to shoulder, another skull mounted at the point they met over his massive torso, the shoulders decorated with more hides and bones and an obscenely huge sword gripped in one massive clawed hand. Two long horns protruded from either side of his forehead, set inline with his glowing, cruel stare and curling majestically as they extended from his skull by a good foot and a half. He was the image of your classical Demon, brutish, burly, horned, hoofed and tailed. When he turned around, two ugly scars were seen on his back, the remnants of where his grand wings had once rested, lost in a disagreement with something far greater than he at the time.
Surveying the camp, the beast strode about, the lesser demons scurrying madly to get out of his way, the workers all diligently at their task, not daring fail lest they incur the wrath of this larger being. It was apparent from their behavior that he held some form of status among them, and given that he eventually settled into the largest structure available, atop a very large seat made of trees and bones, draped with the hides of what prey had been killed, some of which was clearly human, it was clear he must be the leader of this particular horde. The demons had been working with supernatural speed, and it had taken only a few hours for the camp to be completed, food and slaves captured from a close-by settlement already being brought forth for inspection, the brute getting first choice over which he would keep to sate his varied hungers.
As imagined, the screams, cries and general sounds of suffering and woe began to fill the air, along with the stench of burnt flesh, gore and gods know what else. In the course of half a day, the demons had arrived, conquered and now settled in, ready to begin their masters work on the morrow. The portal remained open, kept that way at this end by means of demonic sorcery, a construct set up around it, maintained by chanters that kept up a constant rhythm. Any reinforcements or information would have to come by the portal, so keeping it open was far easier than trying to create a new tear between realms each and every time something was needed. The portal and chanters were under constant guard as well, demons with specialties for such work being kept posted at all times, much like the rest of the camp. It was all fairly and surprisingly coordinated, stark contrast to the crude and primitive layout of the camp. Just because they were demons, didn't mean they didn't have a sense of order in their own right.