Warring Guilds and Rogue Assassins Plague the Streets of AD&D

Thieves guilds and assassins guilds are ubiquitous in AD&D cities and towns. Both assassins and thieves are found on the DMG's city/town encounters matrix:

The assassin entry tells us that not only are cities and towns assumed to have a Thieves' Quarter, but also that (many) assassins may be encountered "at the guild":

The thief entry tells us that "other thieves encountered will be on guild business":

Thieves can "build a tower or fortified building of the small castle type for their own safety", but only within or very close to a town or city:

This calls into question why thieves are listed as potential masters of castles - including medium and large ones - in Appendix C within the procedure for generating fortresses in uninhabited areas:

...but this is certainly not the only contradiction between the PHB and DMG, and isn't particularly relevant to this post, so I won't dwell on it.

What is relevant is that if a thief does build a stronghold, they may use it as a headquarters for a gang, and in so doing attract thief followers. However, if they do so they will "bring the enmity of the local Thieves Guild". This will result in a war which ends "only when and if all the Master Thieves on either or both sides are dead, or if the thief character removes to another locale" (where, presumably, there is simply another Thieves Guild to contend with):

Similarly, the PHB tells us that "most towns and cities" are home to an Assassins Guild which "controls an area of from 10 to 100 miles radius around the headquarters town or city", and that "all non-player assassin characters are members of such guilds". A player character assassin who is discovered in a guild area will be invited to join the guild. They may decline, but will be sentenced to death if they perform an assassination while not a member of the guild:

Later, we are given more information about the nature of the Assassins Guild headquarters, which is "always within a large town or big city", but must be "nondescript" and safeguarded so as to avoid "attention and unwanted notoriety":

Compare that to this paragraph from the section of the DMG on poison, which describes the Thieves Guild as "an accepted part of communal society", so long as they stick to doing things that thieves do. The Thieves Guild can provoke the enmity of the Assassins Guild by engaging in acts which bring unwanted attention (or unwarranted blame) to the latter, or which are seen to step on the latter's toes:

This suggests that while the Thieves Guild may be at war with an upstart gang outside of their organization, and the Assassins Guild may condemn to death an outsider carrying out unsanctioned killings, the two organizations may also be in conflict with each other.

Unlike the thief, who can simply start a gang but must then go to war with the local guild, an assassin must assassinate or otherwise kill the local Guildmaster Assassin to attain 14th level (and later do the same to the Grandfather of Assassins to attain 15th level):

The headquarters of the Grandfather/Grandmother of Assassins can take a variety of forms but, unlike the local guild headquarters, is obvious and "must be located well away from all communities":

(Here we have another inconsistency with Castle Sub-Table II.B. That table lists the assassin as a possible master of the castle, but they can only be 14th level. 14th level assassins are Guildmasters, though, and their headquarters "is always within a large town or big city". As Grandfathers/Grandmothers - 15th level assassins - are the only assassins who build headquarters in uninhabited areas, it would be more accurate for the assassin on Castle Sub-Table II.B. to be of 15th level. But I digress yet again.)

The local Guildmaster Assassin commands 7-28 lower-level assassins. If the Guildmaster is killed, 75% "leave the area" (presumably to defect to a Guildmaster in another city or town) and new (1st level) assassins eventually arrive to fill the ranks:

The DMG goes on to state that the Grandfather/Grandmother of Assassins always has 28 followers of 2nd through 8th level, plus 4-16 of 1st level, and that when they are overthrown, as with the Guildmaster, 75% of these followers desert:

Since Guildmaster Assassins are referred to as "the local Guildmaster Assassin", whereas the Grandfather/Grandmother of Assassins is referred to as "the Grandfather/Grandmother", I'm going to conclude that there is only one such Grandfather/Grandmother of Assassins in the entire world, similar to The Great Druid:

It is my conclusion then that those assassins who desert when the Grandfather/Grandmother is supplanted do not return to the guild or defect to another but instead become masterless rogue assassins who wander the world, employing their skills in unsanctioned killings (thus making themselves persona non grata within the newly established order). 

They may even plot to overthrow the new Grandfather/Grandmother (the player character) once they've accumulated sufficient power. This would require reengaging with the guild structure which, since they've already deserted, may not be the intention. In any case, the player character assassin would be wise to closely monitor any new Guildmasters within their domain!

The assumption that there is only one Grandfather/Grandmother of Assassins and that assassins who desert in the wake of one's thus killing leave the guild forever is at odds with the previous declaration that "all non-player assassin characters are members of such guilds", but I've already demonstrated Gygax's tendency towards contradicting himself. Gygax may also simply be describing the status quo at the time when campaign play begins, not accounting for every possible action the player characters might take during the course of a campaign.

Whether this is another contradiction or I'm simply over-interpreting or making a false assumption, the idea of rogue assassins roaming the world is at least a fun one with interesting gameplay implications, so I'm willing to accept it at face value.

All of this, in fact, is highly gameable information. A player character thief may start a gang and provoke a war with the Thieves Guild. A player character assassin may decline to join the Assassins Guild and be condemned to death for unsanctioned activities, or join the guild and climb the ranks to Guildmaster or even Grandfather/Grandmother and so have rival (and even guildless) assassins plotting to kill them. But also keep in mind that anything the player characters can do, NPCs can do too.

Whether or not there's a player character thief or assassin in play, these organizations are assumed to be present in all AD&D towns and cities and will be operating in the world on their own terms. The player characters may be unlucky and run into thieves or assassins in the streets of the town or city, but perhaps they can also help out the local guild by toppling the upstart gang or capturing or slaying the rogue assassin. Perhaps the players help the upstart gang of thieves or mistakenly hire a rogue assassin, thus earning the enmity of the respective guilds. Perhaps they employ poison in some high-profile skirmish in the city, and now the Assassins Guild is out for their blood.

In summary, pit your guilds against each other for control of the streets, and let your player characters get mixed up in the conflict or caught in the crossfire. Remember that all dealings with thieves and assassins are wrapped up in layers of intrigue, subterfuge, and esoteric rules. The guilds are everywhere, and something as simple as helping or hiring the wrong person or putting a bit of poison on your blade may be seen as the opening salvo in a protracted war with AD&D most dangerous organizations of burglars and cutthroats.