Space Hulk - Ascension has been delisted years ago and is essentially abandonware now. The description very much does it justice and the screenshots give a good impression of the dark atmospheric look.
Gameplay is slow, methodical and often repetitive, but Ascension introduces various additions which help keep things interesting. DLCs added many more chapters to choose from on top of that and adjustable difficulty is yet another way to customize the experience.
Its main strengths lie in atmosphere and good sound design and the relatively small margin for error that lends the player's actions weight and keeps the tension up.