I was surprised to see that this wasn't already suggested - I checked after watching the latest episode of Machinima's Freeman's Mind, on youtube. Sure, it's not horror (except maybe that damned part in Ravenholm). But Half-Life games are pretty damn awesome.
Do I really need to introduce the Half-Life series? store.steampowered.com/sub/715/

I'm pretty sure Pewdie has those games already... though that means he's played them already. Perhaps playing on the highest difficulty and trying to get some of the silliest achievements (gnome, I'm thinking about you) would give him a proper challenge?
So that would be Half Life, Half Life 2, Half Life 2 : Episode One, Half Life 2 : Episode Two, Half Life 2 Lost Coast (short but sweet, and pretty besides).
ALSO, there's Half Life : Blue Shift and Half Life : Opposing Force which could be interesting, since they are a different take on the original Half Life game's story... sort of.
Do I really need to introduce the Half-Life series? store.steampowered.com/sub/715/

I'm pretty sure Pewdie has those games already... though that means he's played them already. Perhaps playing on the highest difficulty and trying to get some of the silliest achievements (gnome, I'm thinking about you) would give him a proper challenge?
So that would be Half Life, Half Life 2, Half Life 2 : Episode One, Half Life 2 : Episode Two, Half Life 2 Lost Coast (short but sweet, and pretty besides).
ALSO, there's Half Life : Blue Shift and Half Life : Opposing Force which could be interesting, since they are a different take on the original Half Life game's story... sort of.