General headcanons for outerhorror? 0: that one sounds interesting!
I'll mainly give my headcanons for the general setting and save the majority of the character headcanons for character specific asks.
The setting: the monsters of outerhorror were exiled to their home planet's distant Ebott moon after losing the war with humans. A thin atmosphere is present that protects the surface from some of the harsher forms of radiation present in space but at the time of exile the moon was considered a non-habitable Class H plantoid.
After many millennia a human passed through leaving the monsters devastated, their king who had lead them dutifully from the beginning now forever gone. To make matters worse only a few short years later an asteroid made impact with Ebbot moon's surface, the resulting shockwave and debris resulted in major damage to Homebase 2. This event caused severe disruptions to the monsters food supply creating a system wide famine and a state of emergency to be declared. Now monsters live in a constant state of uncertainty, scavanging for resources and keeping watch for any sign of danger. In their current state one more disaster could be the end to monsterkind as we know it. For the moment though they depend upon each other in hopes of seeing a brighter day.
Homebase 1 (the ruins): The initial homebase of the monsters this biosphere habitat has long been abandoned by the majority of monsterkind. Many areas are completely inaccessible, the life support long ago turned off and the airlocks sealed. With barely any monsters left in HB1 life support systems are only active in a handful of places, the bare minimum needed to sustain life.
Homebase 1 was the least affected by the impact that devastated the surface of Ebott moon. However it did not escape the effects of the famine that followed. The monsters who had already lived a simple life were faced with a bleak future and adopted a Spartan way of life in order to survive.
Homebase 2 (new home): When monsters began to outgrow the confines of Homebase 1 a brand new habitat was built by order of the king. This habitat was designed to be expandable and has since grown to be quite large as the years went on and new habitat modules were added. Currently Homebase 2 is divided into five major zones. Each zone is covered by a biosphere with the majority of each zones living areas being located beneath the moon's surface. These excavated areas are known collectively as the Caverns.
Zone 1. The Snowglobe (snowdin): The Snowglobe was the first biosphere to be built and is the only habitat still connected directly to HB1, although, the airlock has remained sealed ever since the last human came through. When excavation below the surface began the habitat was discovered to have been built upon a series of icy caves and since then the biosphere has maintained a frigid climate. Monsters predisposed to these conditions quickly made a home for themselves there. After the impact the systems that kept these conditions stable began to deteriorate. Temperatures dropped and ice build up became a common problem in many areas. Traversing through the Snowglobe became a dangerous endeavour and with the strict rationing in place many monsters could not afford to expend unnecessary energy. To the untrained eye it looks like little more than a desolate frozen wasteland.
Zone 2, The Whirlpool (waterfall): The natural springs of water uncovered beneath Zone 2 were an unexpected blessing to the more aquatic monsters. However despite efforts being made to control the water's flow they were never able to fully cease the formation of whirlpools and occasional water spouts. The area soon became named after the former as a warning to those unfamiliar with the waters strange ways. Where the water originates from remains a mystery, but where ever the source may be at some point in its journey the water picks up pieces of space junk. This junk regularly rises and sinks within the Whirlpool's waters and many monsters became fascinated with the various human paraphernalia that would surface amongst the debris. After the impact the systems that had been built to control the Whirlpool's waters took the brunt of the damage. Without these systems the formation of whirlpools and spouts became not only more frequent but more unpredictable. Monsters had to learn quickly how to spot the signs of a sudden change lest they risk being swept away with the waves of swirling space junk.
Zone 3, The Hotspot (hotland): Unlike the previous zones this habitat is not built under a biosphere but instead immediately began with excavation inside a nearby mountain. Any area exposed to the surface was sealed with the same sturdy glass panes as a biosphere but from the outside the Hotspot looks more like a glass antfarm built into a mountain than a upturned fish bowl. The aforementioned mountain however turned out to actually be a dormant volcano with pools of hot magma dwelling below. These pools soon became Homebase 2's prime power source after the creation of Zone 4. The Hotspot took the least amount of damage during the impact. However despite the majority if its internal structures remaining stable the region has since had to deal with rising and falling magma levels. There are fears that the volcano may reawaken and should this happen Homebase would be divided in two. Solutions to this potential disaster are still being researched.
Zone 4, The Core (the core): The Core is the proverbial heart of the habitat, the source of almost all power for HB2. With its massive geothermal generators and vast halls of monitoring systems and cooling vents the Core is the final link between Zone 5 and the rest of the habitat. From the surface this zone is completely hidden, a behemoth of a machine that's runs day and night. The damage it sustained during the impact was devastating and it took every capable monster working together to salvage what they could. Working at barely a fraction of its previous capacity every monster was effected by its loss, like a ripple the damage would only spread with time.
Zone 5, Command Base (the capital): Command Base is the final habitat, the last stop on your journey through the kingdom of monsters. Beneath the grand curve of its biosphere sits the castle and beneath that the vast network of caverns the monsters call home. Once it was home to a beloved King, now the reluctant home of a newly appointed Queen. Like it's new Queen, this habitat weathered the storm and came out the other side with only minor damage. In the face of tragedy it stood firm and inspired its citizens to band together. As the name suggests this place issues the commands necessary to run a kingdom, but it also commands respect and loyalty. Despite the harsh new conditions the monsters now face as long as this place stands they will too.