Okay, so this is a niche where I feel like quality really varies. The thing about 'Dead Space' is that the horror is so environmental and psychological that it's hard for writers to capture that constant dread. You get a lot of fics that just pile on gore, mistaking viscera for atmosphere. The ones that work for me are the ones that lean into the isolation and the slow, creeping wrongness of the Marker's influence.
I kept a list from my last deep dive. 'Signal Degradation' on AO3 is a standout—it's an original character story set on the Ishimura before the outbreak, and the author has this incredible talent for making the routine maintenance of a mining ship feel sinister. You just know what's coming, and the waiting is awful in the best way. Another is 'Echoes in the Static,' a crossover-ish tale with 'Event Horizon' elements that explores what happens to a rescue team finding a 'quiet' Marker. The horror is more cerebral, about the corruption of memory and intent.
Avoid the romance-heavy 'Isaac/Stross' stuff if you're purely after horror; those tend to focus on trauma recovery, which has its place but isn't the visceral scare-fest. Honestly, your best bet is filtering by the 'Cosmic Horror' and 'Body Horror' tags on Archive of Our Own and sorting by kudos. The shorter one-shots often pack a more concentrated punch than the epic-length works.