(The story for) Very Cool Gun (VCG): A Rougelike Shooter with Only One Gun

VCG's Universe

VCG's plot takes place on "Sheol," a strange planet hidden in the depths of space, ominously named after the Jewish underworld for reasons long forgotten. Sheol is a barren, rocky, and desolate planet with an oxygen atmosphere, having once been the home of an advanced alien species who have long since largely died off, though their few remaining members still live deep within the planet's depths. The planet has an extremely unique geological structure, it's comprised of a multitude of extremely porous layers that all rotate in separate directions, forming vast interconnecting cave systems that constantly change layouts as a result. The layers are all extremely diverse from each other, some are rocky caverns, others verdant underground forests, others icy caverns, others still being ruins of the original inhabitants' civilization, sporting strange, angular, and metallic architecture, as well as seemingly endless monuments to some deity simply called the "Guardian," a godlike creature residing at the very center of the planet that keeps guard over its true core, a black hole. It's not known how Sheol even exists as it does, almost everything about it should be scientifically impossible, though it's theorized that the inhabitants "built" the planet to be the way that it is, perhaps with the Guardian's help.

Aside from Sheol, VCG's overarching universe takes place within an interplanetary human society. Humans by now are significantly advanced, having FTL-capable travel and long-established societies across many planets. Each planet was largely settled by a single country or group of countries from Earth, so as a result they're all self-governed and the people from each planet largely share ethnicities and name schemes, though they've all mostly adopted English as a universal language. (Here's where the to-be-explained stuff starts coming in) For example, Yuko's planet (Kōmyō) was settled by the Japanese, Kalista's (Drugomir) by the Russians and Scandinavians, Forrest's (Santiago) by the South Americans, Booker's (Melius) by the United States, etc. This society is in the middle of a losing battle with an unfeeling, cruel empire of alien conquerors simply called the "Out," who want nothing more than the complete subjugation and eventual extinction of the human race. In a last-ditch effort to defeat the Out, the Red Letters, the most powerful and coordinated group of rebels fighting against them, unearth what is codenamed as the "Very Cool Gun," an ancient superweapon of long-since-lost origin that can change its form into almost anything, provided it's given the right easily-manufactured modification. The Out possess a mod that gives the VCG the ability to, among many things, destroy entire planets in a single shot, though they still need the gun itself. The Red Letters hastily assemble a team to take the VCG to Sheol and destroy it by throwing it into the planet's core (Sheol being the only known safe planet with an easily accessible core), using Kalista's "rewind" technology to allow for repeated attempts due to Sheol's extremely hostile insides. However, the Red Letters didn't spend very much time checking who they picked, so the team they assembled is a hodge-podge of complete strangers from totally different places and backgrounds, many of them marred with personal trauma, whether it be from invasions by the Out, the squalid conditions of their home planet (like Kalista's, which has a communist government in place that is slowly falling apart), or their service with the Red Letters.