R

Revan - as seen in Star Wars: Knights of the Old Republic
Revan is unique in this list in being a villain you never actually meet, let alone fight, and whose identity is unique to each player. This monstrously evil Sith Lord, the aftermath of whose galaxy-wide onslaughts you keep stumbling across is, in fact, you before you lost your memory. (We did say there would be spoilers). The gradual build-up to the revelation is masterful, clues scattered throughout the game and influenced by how close to the dark side you play. Once the truth is told, you can choose either to continue Revan’s dark work or to fight for a better tomorrow - either way, he/she’s a villain that can never actually be defeated.

S

Shodan - as seen in System Shock 1 and 2
Artificial Intelligence SHODAN is smart, deceptive, cutting, arrogant and amoral in ways other villains can only dream of. Not purely evil, but rather utterly convinced of her own awesomeness, she means to eradicate humanity just because she finds it lacking. And with no permanent physical form, she’s not a foe who can be vanquished by traditional videogame rules. Much of her ongoing appeal is a result of her having a story arc spanning the two games - in the System Shock world, heroes come and go, merely bit parts in SHODAN’s story. One day, she’ll fix the universe, and it damn well won’t be to anyone else’s liking.

Stitches - as seen in World of Warcraft
Unlike Onyxia, Stitches spawns to terrify junior players rather than to be defeated as a typical boss. He’ll sporadically appear on the roads of low-level Duskwood: a gigantic undead Abomination who’s an eye-watering (at the time) level 35 elite, pretty much the biggest, scariest thing in the game at that point. When he’s on a rampage, you’ll know it - the town of Darkshire plunges into panic, guards sounding the alert and going after him en masse. In WoW’s early months, vast packs of newbies gathered to take the beast down, though tragically he doesn’t see the same kind of action nowadays as we’re all off frying bigger fish in the Outlands.