thefutilist
Adventurer
This is what is of course easily possible in low or no myth approaches, thus low myth is most powerful tool in the railroader's arsenal. But not all GM inventions are such contrivances. If the capability and nature of the enemies or the layout of the location were independently (and hopefully previously) established then it is not a contrivance; they are not created as a response to the player action declaration for the express purpose of nullifying it.
HOW TO HIRE ASSASSINS
I won't bash no-myth here but there is an issue. The issue is that the following three things are radically different at the deepest level (1) Adventure path play (2) no-myth play (3) Situation play. The s in situation is to denote a technical term, it has a very specific meaning in this context.
SITUATION PLAY
Situation play means we begin play with a situation, this is before any scenes are even described. The situation consists of people, things, places that have fictional positioning toward each other, before play begins.
The aim of play is to see how the situation changes into a stable state by seeing how the fictional positioning changes, which happens in scenes. I really do mean the aim of play, everything is downhill and judged by how we change the fictional positioning of things relative to each other. The aesthetic pay off is seeing these things change in relation to each other. We're excited to see how things, pre-existing things with fictional positioning, change in relation to each other.
Which gives this play cycle
1) Situation (the entirety of the game state but distinct from setting) >
2) Scene framing (takes the stuff from situation and puts it into a scene where the positioning may or may not change due to conflicts of interest amongst the characters)
3) The scene has conflict or not and the fictional positioning of this part of the situation changes or not
4) The situation has changed and this new situation becomes the next 1
1) Situation
2) Scene framing
and so on until the situation is resolved. In other words there is no longer 'tension' amongst the various components that are fictionally situated.
CONTRIVANCE IN DIFFERENT MODES
So in situation play, someone has hired some assassins. These aren't part of the initial situation (or maybe not) but must be a logically extrapolated extension of it based on the person doing the hiring and their broad fictional positioning toward the setting.
This is also where theme is expressed though fiat and situational constraint because 'what type of assassins does that person hire?'
You extrapolate based on (1) their priorities/personality (2) what's actually available in the setting which can be fairly broad (3) the resources, positioning of the person doing the hiring.
Jackson has deep roots to the criminal underworld and loyalty matters to him. He's precise, a good judge of character, not massively wealthy. His assassin is someone who is genuinely loyal, deadly, patient.
Bellow is rich, dumb and impressed by trinkets. His assassin is good because of the wealth but also a braggadocio and not as deadly, loyal or patient as Jacksons assassin.
A different player, given the same fictional material to work with, would make a different type of extrapolation. The type of extrapolation made, the causal connection. That's the GM addressing premise. This type of person gets this type of assassin.