Dr. Mercer got a crew member to use as a live subject—probably that Harris guy, who was one of those affected by the widespread dementia down onplaguing the colony, and was then brought on board the ship for study.

"down on" isn't a collocation that I know of. I suggest "plaguing".

He seemed like an interesting one for such a purpose.

You write "He seemed X" when X is an adjective, but "an interesting one" is a noun.

Well, whoever it was, had a hole drilled into their skull, right in the forehead area, and a regenerating sample tissue inserted into the brain.

Alternatively: "Well, whoever it was, they had a hole"

Such an experiment mutated the subject into a new type of Necromorph, known as thea Hunter.

[...] That thing can’t seem to die,; no matter how many times you shoot its limbs off, they just keep regenerating.

Dr. Mercer got a crew member to use as a live subject—probably that Harris guy, who was one of those affected by the widespread dementia down on the colony, andwho was then brought on board the ship for study.

"and was then brought on board the ship" sounds like you're talking about Dr. Mercer, while "who" refers to Harris.

He seemed an interesting one for suchcandidate.

Well, whoever it was, had a hole drilled into their skull, right in the forehead area, and a regenerating sample tissue (inserted?) into the brain.

Missing a verb here, I made a guess at inserted.

Such an experiment mutated the subject into a new type of Necromorph, known as (the / a) Hunter.

"the Hunter" would be singular, but when you say "a new type" it feels like there's going to be more than one which makes "a" a bit more appropriate.