I've got all the Thieve's Guild products. They are really nice. I think the system is a little heavy for my tastes these days, but there are lots of good ideas. Sadly, the complete system has never been published, nor was the 3rd module for the city Haven ever published.
I re-purchased these, and yes, they are heavy for me these days, as well. Never played with any of it, but great, great ideas. Suitable for adaptation to your regular campaign.
Running: two 1st ed AD&D games -- one in Tekumel, the other running the Classic Dragonlance modules. Also running Classic Traveller. Playing: 1st ed Warhammer Fantasy RP The Enemy Within. FASA Star Trek. FGU's Lands of Adventure, run by the author, Lee Gold.
Since I have already done the work, crossposting from TheRPGSite:
Thieves Guild occupies a space between being an AD&D hack and being its own game; it is obvious that the creators built it by expanding on the core AD&D framework, and there are innumerable artifacts of this heritage (e.g. the ability of thieves to use scrolls). The system occupies 33 pages in the initial TG installment, with the understanding that later products would fill out unexplored areas, which they did - if you buy an installment, it is going to be a grab bag of optional rules, modular encounters and small scenarios (usually around a common theme). For example, Installment 2 adds more weapon types (with adjusted damage ratings), combat rules, guidelines for hiring and maintaining hirelings, bandit and highwayman encounters, and The Tombs of Shale-Chuun scenario, which combines the mini-dungeons of Keep on the Borderlands with the traps of The Tomb of Horrors, and is a cool, flexible set of adventures. The approach to the rules is extremely modular, and it would be relatively easy to rebuild them to one's liking.
Ability scores include Strength, Dexterity (subdivided into Coordination and Reflexes), Stamina (also used to derive Magic Resistance), Intelligence (subdivided into Discretion and Talent) and Attractiveness (subdivided into Appearance and Magnetism). Abilities are rolled on 3d6. Races are a mixture of tolkienesque and fairy tale: humans, elves, half-elves, dwarves, hobbits, kobolds, orcs, uruk hai, half-orcs, goblins, pixies and centaurs are all playable. Additionally, random social background, modified by race, is a part of the game, influencing starting wealth, as well as weapon and non-weapon skills. Damage uses the contemporary 3rd party abbreviation HTK (Hits To Kill); however, HTK is not a function of class, but an average of Strength and Stamina further modified by a table based on the total of these two attributes (e.g. if the total is 10 or less, the character gains +1d4 HTK; if it is 10-19, it is +1d6 etc.). There are separate saving rolls, which are actually roll-under ability checks rolled with 2d12; they are also used for non-combat task resolution. Combat: the game uses HAC0 (a precursor to AD&D's THAC0, which was listed in the DMG but left unexplained until later) to resolve hits; weapons have different HAC0 ratings, daggers being easier to use than maces or larger swords. Combat actions take place simultaneously, and there is no initiative, so a character who suffers a death blow may still strike back. There are critical hits and fumbles resolved with a relatively simple chart, and thieves are given additional combat maneouvres (backstabbing, coshing (KOs), poison use and striking from concealment. Armour works as in D&D, but the base value is 0 and goes up, so quilted cloth is AC 2, leather AC 4, chain AC 6, scale AC 8 and plate AC 10. Additionally, armour absorbs a small amount of damage (1-3). Skills: there is a wide range of skills associated with social background; characters must spend double skill points to purchase skills above their social standing! The skills of Thieves Guild are much more socially oriented than D&D's, and include a loving attention to roguish pursuits (which are treated separately, so any social class can learn them without penalty). Magic: a full magic system is absent from the initial installment, but subsequent modules add it gradually - in the scenarios of installment 1 and 2, there are example spell lists which work on a spell point basis, and are probably more mundane and less interesting than the exoticism AD&D occasionally went into. Experience: experience is awarded for combat, the sale of stolen goods (very cool!), successful ability rolls and success during encounters (with success conditions set by the DM). Gaining levels results in the occasional extra hit dice (e.g. one is gained at 3rd, 5th, 7th, 10th and 12th level), hit probability (+2 per 4 levels), dodge bonus (to AC, +1 per 3 levels) and the increase of thief abilities. As a neat way to treat NPCs, Thieves Guild stat blocks classify them as Green (LVL 1-2), Intermediate (LVL 3-5), Veteran (LVL 6-7) and Elite (LVL 8+). Further guidelines focus on fields thieves may find interesting - these are separate subsystems for disguise, fencing, traders, a treatise on guild membership and structure, a justice system (with a huuuuuge chart to determine outcomes; however, Judges Guild's similar guidelines are more colourful).
There are allusions in the books to parts of The Fantasy System that would be presumably released later (and might have been - I only own the first two installements) - classes such as Archers, Centurions, Healers, Mages, Priests, Priestly Mages, Illusionists, Assassins, Troubadours, Traders Hetearae etc. are mentioned but not detailed.
An NPC stat block for Thieves Guild may look like this:
Torban the Turbulent: VET HU FTR [Veteran Human Fighter]; ST 18, CO 16, RF 16, SM 17, MR 11; Weapon BDSWDe [broadsword wielded with expertise] HAC0 4, Damage 3d8, L BOWe -2/2/6 1d6, Spear 3/6 2d8; AC C/S [Clothes/Shield]; HTK 44; WRHOR [Warhorse].
In conclusion, it must be added that the scenarios and playing aids of Thieves Guild are done with obvious care and understanding, although they are not typical D&D fare - better suited to a game with more emphasis on social simulation and social encounters than improbable dungeon crawls. For example, the first installment has very detailed encounter modules with merchants, locales for cat-burglary (one set in the Street of Silk Veils, a red-light district; another in an area of magic and curio shops) and random encounters.
The production values of Thieves Guild are spartan; first printswere supposed to be three ring-bound, and were laid out on some primitive computer (most likely); reprints are in the shape of small booklets. However, they are also extremely dense with material and more material. Secondly, the artwork by Janet Trautvetter, if amateurish, is full of character and a human warmth that is missing from RPG art; her depiction of sensibly dressed fighting women is a particular plus.
I haven't played TG, but I like the scenarios in particular, and the system has its charms (I'm definitely in the camp of those interested in "all thieves" campaigns).
The missing portion of Haven has been an off-again, on-again, off-again project that Tadashi and I have tried to work out with the ex-Gamelords folks, but haven't managed to pull it together. If you're interested in seeing it, as well as reprints of the TG rules, be sure to email Tadashi to let him know your interest!: firstname.lastname@example.org
I've played both Haven as a setting and the Tombs of Shale-Chuun as an adventure. My player characters weren't all thieves, I've just dismissed everything about special rules and all the thieves stuff and played them right as they otherwise were, and this went pretty smooth. If only you've heard me sing the Tombs of Shale-Chuun song in karaoke with the House of the Rising Sun tune in the background! Yes, that was part of the written adventure...