All I know about the unpublished FASA Thieves World adventure "T4 - VENGENCE OF MARKEESH" is that it was advertised on the back of Spirit Stones.
- Blue Camel states on the cover that it's from a concept by Robert Asprin and written by Bill Fawcett
- Traitor states on the cover that it's from a concept by Robert Asprin
- Spirit Stones says it's by Bill Fawcett (Sept 82)
- Dark Assassin by Dave Tennes (Oct 82)
Presumably Bill Fawcett was going to write it, although that's by no means definite from the above list. Has anyone ever reached out to him to see if this product exists in manuscript or outline forms, by chance, or do we know any more about it in general?
Good TW summary info @
Thieves' World - Sanctuary - Wayne's Books RPG Reference and
Tome of Treasures :: View Forum - Thieves' World with some additional context at
Thieves' World: Chaosium RPG Supplements (but not really any additional info/details).