Big Bad Evil Guy
The Big Bad Evil Guy, typically shortened simply to BBEG, is roleplaying gamer slang for the primary antagonist of an adventure or campaign. He is the one 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.
Paladins are automatically required to attempt to offer the BBEG mercy, even if they had happily slaughtered hundreds of relatively much less evil minions on their way to the BBEG's lair.