They had the expanse of an endless amount of space which was as far as time and space allows. Yet as more worlds popped up the less space they had, disliking that fact they sought war on all the other realms to claim land as their own. Yet they had a rival race matching and even surpassing their powers, they were known as The Ancients. The firstborn when the dimensions start to appear, they are known as many things all throughout the entire plane of existence.
They are gods, miracle makers, angels, creators, yet benevolent destroyers who can fight off the void and crush entire worlds. Although as said nothing lasts forever, The Ancients and Voidborn were fading away. The void attacked more viciously to sustain themselves while The Ancients crumbling apart went into deep slumbers to regain strength.
So the other lesser races had to fend for themselves against the desperate Voidborn. Some fell under the onslaught and the absence of The Ancients, others survived to tell their story. Each time the Voidborn attacks a world, there are a few ancient Voidborn called anchors who anchor themselves into the world's fabric of existence. Thus the Voidborn can cross through the barriers of reality, the oldest and strongest of them all is Gale the queen of misery, pain and darkness.
As time passed The Ancients became mere myths and were slowly forgotten amongst the sands of time. While the Voidborn were feared and revered by all the worlds. A thousand years before the fall of Nirvania, the prophecy of the Rising Phoenix was recited by a seer. At that time it was ignored by some but others took heed of the prophecy and prepared for the fall thus these walled cities were borned.
The underground was crowded and was filled with thugs with little to no military support. It was the definition of the survival of the fittest, the underground was also the most dangerous. A species of Voidborn are able to tunnel through the ground and when mining someone can stumble upon these tunnels. An incident such as this will often prove destructive and the creature can only be stopped by the inhabitants of the underground as the military does not care about what happens down there.
The only thing they care about are the supports of the city above which are well defended. The rest above ground do not have to worry about this specific breed of the Voidborn for many reasons. They are unable to survive the sunlight unlike other breeds as the light burns them to ashes. The bedrock lining the underneath of the city is too tough for these creatures to burrow through and the walls too tall to climb over without being killed till… recently.