The term 'first-person shooter' has encompassed quite a range of styles over the years.
Originally associated with the high-speed arena shooting of Doom and Quake, the 2000s saw a shift in the genre to the cinematic cover-shooting warfare that the Call of Duty series is known for. And now, in the present day, we've ended up with a mix of both!
With this fluctuation in the genre's goals, it's easy to assume that the question of accessibility might come into play here too. For example, as far as difficulty goes, the increasing focus on multiplayer must mean a game is only as hard as its players are skilful, right?
Not always, as it happens. Whether playing solo or in a party of hundreds, some gamers yearn for a much greater challenge; a fearsome gauntlet of gunfire that will bring out the bloodthirsty psycho in you and give you that much-awaited rush of adrenaline when you overcome a brutal obstacle.
It's true that high difficulty isn't quite the player repellent it used to be, and the games on this list are a genuine testament to that. Do you have what it takes to make it to the end of these merciless massacres?
9. Tom Clancy's Rainbow Six: Vegas
In an opposite move to that of Call of Duty, 2006's Rainbow Six: Vegas is more geared towards the single player experience, with a campaign revolving around Rainbow operatives being called in to Las Vegas to stop a series of terrorist attacks.
The gameplay of the Rainbow Six series has always mixed careful and tactical strategising with a healthy dose of action, but it's in Vegas that the perfect balance finally seems to have been struck.
In other words, you won't be jumping from buildings into clumps of enemies with bullets flying in every direction, but neither will you be able to spend the whole time hanging back to weigh up your options. There's a realistic kind of motion to the game that forces you to be vigilant at the same time as being unafraid to get pulling that trigger.
Which, of course, means it's going to take quite a while to get the hang of. You've got to put real effort into pushing forward in this game, and perhaps the other shooters you've been playing up until this point aren't the best basis for this new playstyle.