This scenario can be played as an addition to your current adventure when players are traveling. This D&D game encounter takes place at an abandoned shrine to a forgotten thief, possibly an avatar of some evil, well-known god of thieves. A group of 2-6 characters is recommended, each of 3rd-5th level. A cleric would be extremely helpful, and an assortment of magical weapons is necessary. This scenario may be dropped into an ongoing campaign, as the player characters are traveling along a minor road in a forested region. The time is late in the day.
With a little imagination, converting to DnD 5e, Pathfinder, etc. should be very easy for any Dungeon Master, but if you need help on conversion to 5e, I created this page for such an occasion: Convert Old Dungeons and Dragons Adventures To 5e