I Am the Last of My People
Cataclysmic events are pretty standard for many role-playing games, but when it happens to your character, it can significantly alter how they view and interact with the world around them.
In horror games like Call of Cthulhu, losing your entire community and culture through this type of cataclysm could mean the difference between keeping your sanity and being driven into insanity, or the difference between helping to save the world or helping to destroy it.
In science fiction games like Star Wars RPG, perhaps you were off-world and your planet was destroyed by one of the Death Stars leaving you as the last living member of your entire planet.
In fantasy games like Dungeons & Dragons, maybe it was an ancient red dragon like Thordak, the Cinder King (created by Matthew Mercer for Critical Role - Campaign 1) who laid waste to the entire druid community where you character grew up as it was escaping from the Elemental Plane of Fire. The loss of your entire community could greatly impact your character's purpose and goals for the rest of their life.
How ever it happens, being the last surviving member of your community (race, village, city, country, planet, etc.) can produce a lot unexpected emotions and/or storyline changes and variations.
A once peace-loving character may now begin a quest for revenge;
A battle-hardened fighter or barbarian may descend into a state of perpetual depression;
A wise Jedi Master on the Jedi Council may begin the path of fear leads to anger, anger lead to hate, hate leads to the Dark Side while working themselves to the goal of becoming a Sith Emperor;
An educated scientist may cease trying to find a cure for cancer and instead begin developing a biological plague to eradicate an entire race of invading aliens only to make a mistake and instead kill off 90% of the indigenous population of the character's planet.
This biggest question to think about when crafting a backstory for this character is to look at how this event may have altered their mental and emotion health. Have they become locked into a continuous state of hate and anger? Maybe they don't see any end to fear and sadness stages of grieving? Have anxiety, PTSD, and depression become your character's new best friends?
By discovering these emotional and mental health issues, you can then begin to unravel how they are dealing with that incredible loss and what they are going to do about it. While crafting your character's backstory you can think about plans and goals that they may have for their future beyond the cataclysm, and then hand it over to your Game Master to incorporate that storyline into the game somehow which will then give you a chance to play out those goals and plans to see how it affects the rest of the story arc and they other players and player characters in your adventuring party.