Big Bad Evil Guy
The Big Bad Evil Guy, typically shortened simply to Big Bad or BBEG, is roleplaying gamer slang for the primary antagonist of an adventure or campaign. He is the one who is ultimately responsible for whatever shit his associates or minions do, and defeating him usually means the successful conclusion of a campaign (or at least a part of it). To ensure that this is sufficiently dramatic, DMs often make the BBEG a powerful character, making this final encounter something of a boss fight. The BBEGs seem to know this is coming, so they will usually construct a suitably impressive lair for the suitably dramatic confrontation to take place in.
Many of these fantasy villains often like to address themselves as 'The Dark Lord' or add 'Dark Lord' before their name, just in case you goblin-fondling peasants weren't paying attention to who they are.
Hilariously though, the sheer incompetence of some of these big bads is at times a beauty to behold, as their over-arrogant natures, the stupidity of their minions, or luck and/or skill on the part of their opposition means their plans fall apart dramatically. For example Harry Potter, despite being a teen and knowing as much magic as a mouse for most of the series, time and time again foiled Voldemort's plans. Despite having control of one of the key factions in 40k, Abaddon has failed to get anything done in 10,000 years (the fact that Abbadon hasn't been killed or overthrown yet is Games Workshop won't let the story go in that direction). And yet how do they get by with so much failure when they would have roasted their henchmen alive if they had done the same? Simple; if you dare to mention it to their face about their joy-to-watch failures, YOU will be the one roasting alive!
In most video games the BBEG is usually the final villain which you and maybe a couple of hanger-ons have to defeat to win the game. Whether it will be a hard fight or not depends how merciless the game designers are feeling.
Paladins are automatically required to attempt to offer the BBEG mercy, even if they have happily slaughtered hundreds of relatively less evil minions without mercy on their way to the BBEG's lair.
Requirements to become a BBEG
1. Kill a lot of innocents (unimaginative, but kind of a must-do on the list; a tradition).
2. Make big grandiose plans that also include big holes in them, just enough for someone to mess up if needs be.
3. Surround yourself with minions and right-hand men. Your evil meter goes higher the more bodies you can pile up there.
4. Have a characteristic to define you from the masses, like a menacing eye (Sauron), the ability to shoot lightning from your hands (Sidious) or an embarrassing track record of failure no-one can mistake (Abaddon).
5. Perfect evil laugh and twisted sense of humour.
6. Have a super bad-ass evil lair.
7. Don't melt into goodness at the sight of baskets of kittens or puppies.
8. Pretend you don't have any possible mental problems even if they are there.
9. Beat up anyone that even looks like competition.
10. Be a dick to everyone, even your own side (tough love).
11. IMPORTANT: Do something memorable. It is not enough to just do all of the above, you need to really do something unique to stand out from your peers. Sidious masterminded the complete destruction of the Jedi and the rise of the Sith to masters of the Galaxy. Sauron helped fuck up his world and make a lasting legacy of evil by creating so many evil races. Vect established the single most unpleasant city realm in the galaxy and allowed the Dark Eldar to flourish as a people. What will YOU do to get your enemies quaking in jealously and respect?