Warhammer 40,000, known informally as "Warhammer 40k" or just plain "40k", is a miniatures-based tabletop strategy game by Games Workshop. Drawing heavily on their previous Warhammer Fantasy game, it began as "Warhammer In Space", but has over time grown distinct from (and far more popular than) its fantasy counterpart.
Thirty-eight thousand years in the future, the mighty Imperium of Man has expanded across the galaxy... to discover that the galaxy is a hell that would make Hieronymous Bosch shit himself in terror, and that it has a hell. From without, the Imperium is assailed by alien monsters from the depths of space, nightmare death-machines and soulless daemons (as well as soulless death-machines and nightmare daemons); from within, treachery, heresy, mindless incompetence and the festering taint of Chaos threaten to tear it apart.
Warhammer 40,000 is not a happy place. Rather than just being Darker And Edgier, it paints itself black, takes a running jump and hurls itself head first over the edge, bellowing "WAAAGH". The Imperium of Man is an oppressive, stark, and downright miserable place to live in where, for far too many people, living isn't something to do till you die, but something to do till something comes around and kills you in an unbelievably horrible way - quite probably something on your own side. The Messiah has been locked up on life support for the past ten millennia, laid low by his most beloved son, and an incomprehensibly vast Church Militant commits hourly atrocities in his name.
The problem is, as bad as the Imperium is, they're not quite as bad as many of the other factions. Death is about the best you can hope for against the vast majority of the other major players in the battlefields of the 41st Millennium. The basic premise of 40k, insofar as it can be summed up, is that of an eternal, impossibly vast conflict between a number of absurdly powerful genocidal, xenocidal and in one case omnicidal factions, with every single weapon, ideology and creative piece of nastiness imaginable turned up to eleven. The basic sidearm of a Space Marine is a fully automatic armour-piercing rocket-propelled grenade launcher. The Astronomican, a navigation aid, has the souls of thousands of psychic humans sacrificed to it every day, dying by inches to feed the machine. The faster-than-light travel used by most factions carries with it a good chance of being eaten by daemons. There are also chainsaw swords, armored gloves that crush tanks, mountain-sized daemonic walking battle cathedrals, tanks the size of city blocks and warships that level continents, if not simply obliterating all life on an entire planet just to be sure. And sometimes even that doesn't work. There is no time for peace, no respite, no forgiveness; there is only war.
And you are going to die.
The 40k universe is a spectacularly brutal playground of tropes and horrible things taken to their absolute extreme, and in some cases, beyond. Entire planets with populations of billions are lost due to rounding errors in tax returns. Orders of capricious, fanatical, genetically engineered Super Soldier Knights Templar serve as the Imperium's special forces, while the trillions of soldiers in its regular armies take disregard for human life further than most people could believe possible. A futuristic space Inquisition ruthlessly hunts down anyone with even a hint of the taint of the heretic, the mutant, or the alien, and is backed up by legions of supercharged daemonhunting super soldiers and fanatical power-armoured battle nuns. The ancient and mysterious manipulator-race contrive wars that see billions dead so that small handfuls of their own may survive, while their depraved cousins cannot live without torturing numberless innocents to death in unimaginably horrible ways. There's a Bug Swarm trying to eat everything in the galaxy, a light-years wide hole in reality through which countless daemons and corrupted daemon-powered super-soldiers periodically attempt to destroy the universe, and an entire civilisation of undying Omnicidal Maniacs serving their star-god masters' desire to exterminate all living creatures, down to the last bacterium. There's a genetically-engineered survivor warrior species infesting every corner of the galaxy and cheerfully trying to kill everything else in the galaxy because it's literally hard-wired into their genetic code. The closest thing to the good guys you can find in this setting is a tiny alien empire sandwiched between all the other factions, and they may or may not have a thing for forcing new subjects into their empire through orbital bombardment, sterilization, and concentration camps, but they will at least offer you admittance to their club. And you may even find solace like that, being, as most of their race, mind-controlled by a few "benevolent" elites. That's your best bet at happiness in 38 000 years from now.
As well as the game itself and its rulebooks, faction-specific, setting-specific and campaign sourcebooks, 40k has spawned a range of spinoff games and publications. Over sixty 40k novels and short story anthologies, including the successful Gaunt's Ghosts, Eisenhorn, and Ciaphas Cain novels, are published by the Black Library, a subsidiary of Games Workshop, who also published the now out-of-print comic book Warhammer Monthly and short story magazine Inferno. Boom! Studios now publish comics set in the Warhammer 40K universe, in the form of various mini-series, rather than an ongoing title. There is even a full-length fan film, Damnatus, which was approved, made, banned over conflicts between British and German IP laws, then leaked online. Spinoff tabletop games include the space combat game Battlefleet Gothic, large-scale strategy Epic 40,000, gang-based Necromunda, all-Ork Gorkamorka, small scale Alien-influenced Space Hulk, RPG-influenced "narrative wargame" Inquisitor, and the more traditional RPGs Dark Heresy, Rogue Trader, and Deathwatch. A small but growing number of 40k videogames have also been made; early examples include the Space Hulk series and a slightly obscure isometric Genesis / Mega Drive game called Aspect Warrior. More recent are Warhammer 40,000: Dawn Of War and its sequel Dawn Of War II, a pair of Real Time Strategy games for the PC; Warhammer 40,000: Fire Warrior, a First Person Shooter; and Warhammer 40,000: Squad Command, a turn-based tactical game. Currently in development are a third-person shooter, Warhammer 40,000: Space Marine, and a MMORPG under the imaginative working title of Warhammer 40,000 Online. An official CGI movie, Ultramarines, was recently announced, following up on a number of live-action shorts shown at various Games Day events in the 90s. Before you start screaming about the former, consider that the script is written by Dan Abnett.