That’s not a split negative here like "ne … pas", "ne … rien" or "ne … personne" but the restrictive expression ne … que so rien is optional here.

Ne … que means "only".

Nous ne parvenions à voir devant nous rien qu‘un abîme fétide.

means something like:

We could see in front of us nothing but a foul chasm.

Note that both devant nous and rien can be removed without breaking either the meaning or the grammar here:

Nous ne parvenions à voir qu‘un abîme fétide.

We were only able to see a foul chasm.