9. The Nihilanth Half-Life
I dont even know how long it took me to complete Half-Life. Frankly, I dont want to know. If I knew how long Id spent pummelling headcrabs, shotgunning vortigaunts and massacring innocent scientists (dont pull that face, you know youve done it too) Id probably fall into a deep depression. But when you play something for that long, theres a certain level of investment that goes into it. And when you know youre tantalisingly close to finishing that journey, any potential obstacle to that sweet feeling of completion will be hated all the more. Enter the Nihilanth. The architect of the Xen invasion of Black Mesa looks all sorts of creepy, and its your task to put him down. But what a task it is mostly invulnerable and capable of transporting you into a room full of aliens, hes the sort of opponent who makes you want to hurt your computer. If the teleportation trick wasnt enough, the big bald one occasionally cuts out the middle man and throws some more aliens at you. Or if hes feeling particularly fruity, hell just attack you himself with high-damage energy blasts. The Nihilanth is capable of trying the patience of a monk, and youll make frequent use of the quick-save function throughout. Then you get sucker-punched with THAT ending. Not cool, Valve.