There were dozens of such series in the late '80s and early '90s. My first guess is the "Lone Wolf" series mentioned above (launched 1984). It has a grimdark flavor; if you remember creepy illustrations, I'd look there first.

Other options on my bookshelf include "Fighting Fantasy" (including the famous "Creature of Havoc") — https://laurencetennant.com/bonds/creatureofhavoc.html

Or "Middle-Earth Quest" (1985-ish) — https://gamebooks.org/Series/270/Show

Or "SwordQuest" (1985) — https://gamebooks.org/Series/333/Show

Or "Fabled Lands" (1996) — https://en.wikipedia.org/wiki/Fabled_Lands