|This article or section is about something oldschool - and awesome.|
Make sure your rose-tinted glasses are on nice and tight, and prepare for a lovely walk down nostalgia lane.
Epic was a system for running massively huge (or, one might say, EPICally huge) games of Warhammer 40,000, using streamlined rules and 6mm scale figures to make things feasible. Forge World never bothers to stock shit for this anymore since it has been overshadowed by its big brother (quite fucking literally) Apocalypse. When Games Workshop switched from metal to Finecast and then plastic, they (and Forge World) declined to update the molds for Epic (and the other Specialist Games), so the stocks are slowly dwindling away. This is purely a game play decision and has nothing to do with the fact that the 28mm scale resin models in Apocalypse are outrageously expensive and supported by special books you have to buy. It is currently being remade. How long has it been "being remade" for though? Basically 40k's Warmaster.
In the Before Time
Epic began as a system by the name of Adeptus Titanicus and was all about giant-ass Titans fucking each other over with huge weapons and stamping on puny infantry. Then, along came a system called Space Marine: Epic designed to allow more rounded armies and hailed as a way to show the true scale of Warhammer 40k conflicts, seeing as the fluff demands wars on the level of the Apocalypse over a Black Templar spilling iced coffee on an Eldar Guardian. As this was during the Second Edition phase of design, it did this via a diseased combination of "Army Cards" where building an army was a matter of creating a spreadsheet with a bunch of dead trees. In this system the majority of models were 1/350 scale, while large vehicles were 1/700 scale to keep them affordable (seriously, there was a time when Games Workshop actually thought like this). This also got GW the Space Marine registered trademark in the UK, which they took great joy in forcing figures based on the Aliens franchise to acknowledge even though the second movie came out first.
Old Epic had some strange issues with armament; specifically, weapons were divided into broad types with no real acknowledgement of different subtypes; all "bolters" (pistol, bolter, storm, heavy) had the same stats, for example, and the Battle Cannon in a Baneblade's turret had the same stats as the one in a Leman Russ Battle Tank's, or the one in a Stormblade's sponson.
How it worked
An Epic army was organised using army list-like groupings based on cards with point values. Armies were built around Company cards, which could have up to five Support cards and one Special card. Company cards were company-level infantry (e.g. Space Marine Battle Companies, Eldar Warhosts, or Ork Clans) which were usually split into three separate detachments on the table. Support cards were additional infantry squads or tank squadrons, and Special cards were the army commanders or superheavy vehicles, Titans and suchlike (think 'decurion' formations, and you're on the right track).
Each of these cards also had a point value printed on it, and as usual armies were limited to a number of points agreed on by the players.
At some point, however, someone realised that Space Marine meant that the average army contained loads of cheap plastic minis and only a few big, expensive lead Titans. This led to the creation of Titan Legions, which introduced Titan Company cards, a pair of new super-Titans for the Orks and Imperium (with more planned) and the Imperial Knights, which entered their 2-year cycle of GW forgetting and remembering they exist at this point in time. It also marked the start of the fluff acknowledging that Orks actually had aircraft.
Titan Legions also introduced greater support for large units of armour as Company cards, and the Tyranids as a playable army; written in concert with Codex: Tyranids for 40K, this was the first time it was acknowledged that any Tyranid creature larger than a Carnifex and smaller than a Hive Ship actually existed.
Predictably this failed to really do anything; Titan Legions was underwhelming and didn't address any of the issues with the core system, instead just adding an extra dollop of complexity on top of it. Most of the new minis were never released at all, while some planned for later supplements like the Ordinatus weapons only ever had their rules featured in White Dwarf. Being the only place the Squats didn't suck made the Epic system an obvious candidate for the chop, and soon the whole line was discontinued.
Some people liked this old Epic with its sub 50 page rules book and have kept the game alive with revised rules, newer armies from later Epic versions, and in some cases killing your printer with PDFs that look entirely too good for a fan project. It is called Net Epic http://www.netepic.org/netepic.html
The Other Other Other Epic
Eventually a newer, streamlined system called Epic 40,000 came out. It was well-designed and the rules made sense, and the players who bought it (both of them) saw that it was good. (Unless you liked Space Marine/Titan Legions, Then Epic 40K was something to hate, even though Andy Chambers and Jervis Johnson have openly called it "the best rules they have ever written"). On the upside the starter box was full of awesome stuff, lots of molds were redone to look good, and having a game with an Imperator Titan or Ork Mega Gargant wasn't like combining Star Fleet Battles rules with Epic. It died in less than two years for a reason. And butthurt Squat players mad their army was entirely removed can't have been enough of a reason alone to cause sales to be that bad. Epic 40,000 can honestly be the line in the sand where Games Workshop began rushing headlong into the completely loathed company they are today. This is of course conjecture from boomer tier fa/tg/uys who hate everything new.
The End of Epic
"Hey, you know what would be awesome, making 40k scale models to match the level of scale Epic was trying to make."
And thus began the slow spiral into madness. Don't get us wrong, we love EXSBAWKS HUEG models and some are simply too awesome to pass up, but Epic was a relatively rules-easy game that had a solid fanbase that died to Apocalypse, which was poorly designed and is cumbersome to play. There was a new system called Epic Armageddon, which GW barely supported with a range of old Epic 40,000 models, and dropped entirely in 2013 with the rest of their Specialist Games. Rumors turned up in April 2014 of a new 10mm Epic game in the works, but not many people took them seriously. They were both right and wrong.
It's coming back!
 Gamesworkshop is now remaking Specialist Games, everyone knows why they're doing it but hey, epics coming back too! Even though it literally half a decade after the first rumors, we'll take what we can get.
 Rules for 30k epic for the NetEpic system.
NetEA: Fan-made rules system based off of Epic Armageddon. Has community-approved tournament lists, as well as many other fan-made army lists.
Imperius Dominatus: Fan-made update to the NetEpic system, but using d10s. Started as a Horus Heresy focused project, but is slowly expanding to other factions.
GW's latest "version" of Epic consists of Knights, Titans and Bigger Titans. Set in 30K, no support for demons and xenos units so far. The rules seem to be based on Imperial Knight: Renegade with the complexity of piloting your Titan! Still fun if you are into mech battles. For aircraft models there's Aeronautica Imperialis.