Related to this question, what is actually involved in a turf war between a high-level thief character and the local Thieves Guild?
I assume that Assassins are involved at some point. What other types of characters or scenarios would occur?
The basic gist is that my buddies are going to kill or beat the crap out of your buddies with the minimum amount of attention from the authorities. This continues until you submit or die. And if you are the leader likely you will die no matter what and your flunkies will be run by a new lieutenant appointed by me.
There is a lot more stupidity going on with turf wars. If they were that smart probably they wouldn't be thieves in a turf war in the first place. The exception is somebody younger who is just starting out. Note this is a general observation that if you look at the everybody involved they are not as organized, not as smart, etc., etc. as say the nobles, merchants, and so on.
However the dividing line between what the thieves do and the rest of medieval/fantasy society does is inches. A war between two barons can have all the elements of a turf war.
What really counts is loyalty to your buddies. It will be the defining element of the conflict. It will start for any number of reasons, greed, lust, or plain stupidity. But it will be fueled by revenge and the desire not to fail in front your peers.
It's also about relative status. Conflicts that are not to the death are fought until one side acknowledges, by whatever custom dictates, the other is better/bigger/badder/boss.
Plus since we are dealing with the criminal element the relationships within and between the faction are distorted, twisted or plain abusive in any number of ways. For example, part of a lieutenant's loyalty to his boss may involve the fact his boss supplies his girlfriend with Lotus Powder (a drug) for free. This connection can be exploited by the other side or be a source of a conflict.
Your character finds this out and destroys their supply of Lotus Powder, making it look like the Boss' screw-up. Now the Lieutenant is unhappy and you come along and offer him an alternative supply for a few "favors".
The City-State of the Invincible Overlord has been one of the centerpieces of my campaign and I run several campaigns where thieves and the criminal class were the focus.
One of the most memorable was when two of my players played a pair of brothers. Thugs for the most part. They were given a job and fed inaccurate information. It got screwed up and they got blamed for it as well as not getting paid. They went chaotic and started with their boss and killed him, then killed their boss' boss and so on, working their way up the hierarchy. They were given some information about the Prince of the Thieves Guild. When they followed up they found themselves in a trap and were killed.
The players weren't upset as they acknowledge they shouldn't have trusted the contact as much as they did. About midway through their killing spree one of the rival Princes, through intermediaries, started feeding them information and using the chaos they caused to advance his position. When their usefulness ended he laid the trap and killed them.
Every fire needs a spark to start it. Have a slight (real or imagined) start the plot-line. Maybe the old and established Thieves Guild does not get the contract to steal the widget. Maybe the Upstarts decide to prove their skills by stealing the flag off of the roof of the thieves guild.
At each step of the way, there's going to be many opportunities to escalate or de-escalate the conflict. For the war to happen, concentrate each side "one upping" the other. Sooner or later, there will be wholesale thieving going on between them. And sooner or later, Agent X will be caught in Guild Y and get killed for it.
This stage should mostly be assassins or thieves killing as stealthily as possible. My guess is that one of these stealth-kills goes sour, and the assassin barely makes it out of the guild headquarters. In trying to escape, they chase him to his hideout, and somewhere the guard sees this happen. You now have an open conflict.
Fast forwarding to the end of the conflict, either one of the guilds gets wiped out (or nearly so), or one of the guilds decides they are in over their heads, and calls for a peace. The details of the peace/truce can make a LARGE number of plot-seeds (guilds split the city, but start getting jobs in my side that takes me to your side, how does that get handled; etc.)
The thieve's guild would probably have contacts with local law enforcement, double agents probably. What could happen is one side would steal something from a prominent figure, and one of the snitches would report that the character had done it. Now the character is the focus of an investigation, and has acquired the enemy of a wealthy and powerful NPC.
The guild would also try to poison the well for the character, pressuring fences to not take on stolen goods, and just generally shutting down the black market. A few would decide to work with whomever gets them money, but then assassins would be sent out to take care of them. It's not just an issue of taking out the person who is at the top, but also making it so that nobody wants to switch allegiances.
The character would have to find a way to fight back, perhaps killing any fence who refuses to do business with them, or paying off a snitch to lay the blame of something stolen back on a prominent figure in the thieves guild. Alternatively, the character might want to keep everyone else out of it and just take out the leaders of the thieve's guild.