Détruittale is caused by a shift in fortune vs misfortune. But what is Fortune and what is Misfortune? Why is Fortune and Misfortune capitalized in this sense? What is affected by this shift?
Fortune is everything GOOD that happens in day to day life. When you wake up and go on a walk and find a $20. When some stranger buys your drink for you. When you have somewhere to go and have no traffic. When you get that 100% on a test. When you get into a car crash but you don’t get injured at all, and your vehicle is somehow intact aside from a few scrapes. These are all examples of what Fortune causes.
Misfortune is everything BAD that happens. That day when you lose your wallet, or your home gets robbed. When you have a car crash and you get broken limbs, or a family member who was riding with you loses their life. When you fail your class, or when you get fired. When someone robs the store you are working at. These are things that Misfortune causes.
The reason why Fortune and Misfortune are capitalized is that there are beings trapped inside the Void that are the embodiment of Fortune and Misfortune. As long as both are trapped inside, then Fortune and Misfortune are balanced, making the life we know as every day life. However, if one escapes, then that one begins to overpower the other. If Fortune were to escape, then more good things would happen then bad, but if Misfortune escaped, then more bad would happen then good. If BOTH got out, then they would war with each other, causing chaos in multiple was, for both overpoweringly good things would happen, with overpoweringly bad things happening. These creatures do not care for anything other then making the things they cause happen to happen. As long as both are trapped, they cannot heavily influence others.
But someone made a mistake, and created a tear in the void. It was small, and nothing could escape at the time, but an influx and efflux of great power temporarily increased the tear long enough for Misfortune to escape the Void.
The Underground and the Surface began to suffer for it.
Most of the look of the Underground itself wouldn't change aside from there would be more things to promote safety of everyone, like supports in certain parts of the Underground to prevent any sort of collapse. There would be fences, and people would either be wearing things for their injuries/disabilities or have someone to help them with a mental issue that developed. Sharp things that aren't a magic weapon would be dulled. Ice on the ground is broken up and has gravel/clay put over it. Waterfall itself has warning signs everywhere to be careful about the wet ground. Hotlands has fences to prevent people from falling into the lava. Any place with a long drop has a fence too.
About 90% of the Underground is affected by this; spirits, ghosts, elementals, or beings more tied to a less physical plane (like beings with a 6th sense) being affected more mentally then everyone else, and everyone else getting affected more physically by bad happenings.
Many people approached Asgore about these events, and he appointed a scientific team to find solutions. Not many people approved of this, and wanted Asgore to do something about it, but Asgore became scary. He wielded a scythe instead of a trident now, and wore a mask to hide his face. The people still loved the King, for he was still kind and caring, but he became withdrawn and didn't come out as often. It made a majority of the Underground scared and worried.
Many people in New Home grew nervous, some even going as far as to move out of New Home and away from the King. They grew scared to do anything that the King didn't approve of, and would insist that everyone always follow the rules and not press the King to do things, or contradict what he says.
Those in Hotlands grew nervous around the lava, and moved to areas away from the edges and away from the Core, not wanting to risk some horrible accident, insisting that safety measured be added as soon as possible. They didn't like how slow things were being dealt with, and grew frustrated and harsh towards those who weren't concerned about what was going on in the Underground.
In Waterfall, you either learned how to swim, knew how to swim, or could fly. Safety netting and basic fences were put in where you couldn't fall in accidentally, but you could still go for a swim if you wanted. The cliffs got fenced and the bridges got some safety rails. The Trash area wasn't considered off limits, but you are advised to wear safety gear before going there. If you couldn't swim, you either moved out of Waterfall, or you moved away from the water. They Understood that certain areas have more dangerous things to deal with, like the Core and Hotlands, but they still wanted more (and stronger) safety supports for Waterfall.
The Temmies became reclusive, only leaving Tem Village is they REALLY had to, and began to use the scientific advancements that were released to grow food. They were still friendly to anyone who showed up, but they didn't want to risk anyone getting hurt.
Snowdin became a more welcoming place to live, being less cliffs and dangerous things like Lava, and the people there being very friendly. The ice was dealt with my salt and clay pebbles so that the clay would absorb water and make everything less slippery. Trees were cut back from the main living area to prevent trees from falling down on someone, and the trees that were kept got supports to prevent it from falling over. Since only a few people living in Snowdin were affected by the disaster (since it started closer to New Home first and they took precautions before anything bad happened), they didn't see what was happening as something that couldn't be prevented, like other monsters. They still had some bad things happen, but not as much as other places.
The Royal Guard gained more jobs. They had to be strong enough to push through the bad occurrences and still be able to help other monsters. They protect and assist all who need it. They still hunt down criminals as well (which, while there aren’t many in the Underground, there are a few), and their main job is to still be on the lookout for humans, but the safety of the monster populace comes first. If a human isn’t doing something wrong, and a monster is in trouble, then it is a guard’s duty to assist the monster first.
In the Ruins, most of the monsters moved closer to the old Royal house. Toriel helped put up supports and cooks food for all the monsters taking shelter away from the crumbling areas of the Ruins. The Ruins became super dangerous to live in, making the monsters living there have to adapt in order to survive the dangerous area. They didn't know anything about what was happening outside the Ruins, and they were unsure they wanted to know, instead deciding to stay there, keeping the door to the rest of the Underground shut.
Any puzzles or traps were changed to be safer in all areas of the Underground.
The population as a whole in the Underground reacted by extreme support for each other and helping each other out. If you didn't help someone when you could have, the first time, you get a warning, but the second time, you were subject to being ignored or locked up for being a liability to other's safety.
The happenings did eventually get a name. They were called The Denouement.
As for how those afflicted by Misfortune, they immediately got support from other monsters. They were very willing to help each other out, and those not afflicted helped as much as they could (while only a couple retreated into areas with no monsters to avoid the risk of becoming afflicted by The Denouement as well).
The Surface however, did not change in the same way as the Underground.
Over time, the people on the surface began to become cold to others, thinking that these bad things were caused intentionally by other people. Things began to get more tense and hostile.
While, no new Wars started, but people refused to help others they saw in need, and began to be more distant with each other. Most other things didn’t change other then things becoming horrible.
Politicians would campaign of trying to make things better, only to fail miserably when they got in office. There were still good Samaritans, and family stuck together, but people pushed those they didn’t know away.
More arguments happened, and less children were being born due to a reluctance to get to know other people. The population swiftly declined after 173X during the rule of Misfortune. The humans have no name for the affliction, thinking that is was just caused by the jerkiness and douchebaggery of other humans.
Now, how bad would things be if Fortune escaped with Misfortune running about?
If Fortune escaped the Void while Misfortune was still running around, they would begin to war with each other. Things would begin to fall apart. WWII would be a baby compared to the death and destruction caused by their fighting. The world would begin to fall apart as towns and cities were destroyed and ruined, and living creatures all over would begin to die. The first and last time this happened, it took 100 Boss Monsters and 500 Wizards to trap them inside the Void, resulting in the death of all those who helped trap them. That was back when there were a lot more Monsters and Wizards, and when science wasn't as developed. To trap them again would be near impossible unless you had someone who knew how to deal with them. It would be the end of the world. People would be scared for their lives. They wouldn't want to die. It is the base fear that almost everyone (besides a few) has, and it would show by them trying to find ways to prevent their inevitable death.
