This is a combi-thread covering multiple games in the "Space Hulk" genre of 40k games.
Space Hulk by Full Control Studios. It's a computerized, turn-based, cogitated version of the Space Hulk board game and it's exceptionally faithful to the original design. Just like the original, you're 100% subject to the success or failures of dice rolls, and even fans found they weren't enjoying that in a video game. Still, if you want the actual table top experience in a video game it is exactly as advertised.
Space Hulk: Ascension also by Full Control Studios. They followed up SH with a direct response to fans saying they wanted a "gamier" version of Space Hulk to play. It's got its own rules loosely based around the SH table top game but made more forgiving and flexible for video game play. Like its predecessor it's also turn-based gameplay. There's customizable Terminator Space Marines you can level up, spec out with different gear and skills, etc....While it may still be a slightly bargain title, I enjoyed it as a sort of 40k take on XCOM.
Space Hulk: Deathwing by Streum On Studios. The same Streum On of E.Y.E Divine Cybermancy fame. It's a FPS, Left 4 Dead-ish take on the genre, where you play a squad of Terminators through a set of campaign story missions taking place in procedurally generated levels of a massive space hulk. There's new weapons to unlock as you play through the campaign, secrets to find in each level, and in multiplayer you can accrue levels and points to unlock different looking armor sets and weapon skins, as well as some slight gameplay tweaking options.
Space Hulk: Tactics by Cyanide Studios. The newest entrant to the Space Hulk series, Space Hulk: Tactics looks to be very similar to Space Hulk: Ascension, just prettier and with a lot more time and money thrown in to development. Also you get to play as the Genestealers if you want.
The thread may read a little confusingly because it's changed focus and narrative a couple of times. But no one ever said doing the Emperor's work was going to be easy.