Let's say you're an Amneni assassin who needs to kill someone with poison. How do you do it? I can see a conflict to sneak into the person's house or to pass as a servant and poison the wine, but then? What do you roll to see if the victim his dead or not? And if he extends conflict?
Thanks for your attention and answers.

Ooh, ooh, I know this one! We had this exact discussion a couple of times in the old CRNgames forum, which I read religiously last spring while writing my new TSoY book. Let's see... this and this are about that, at least. Good answers there.

When I play, the deathiness depends largely on the sort of crunch we're using to depict the poison. That is, the mechanics provide the poisoner with some mechanical guarantees for what he might accomplish. If you don't have any crunch to back you, for instance, then it's a simple Ability vs. Ability check to poison another. Which Ability it is would depend on the turning point of the narration - might be some sort of disguise or spying or servant Ability, for example, or something to do with lying, etc. Or herb lore vs. the other character's constitution for that matter.

As for specific poison crunch, those things usually provide their own procedures for how to go about it. If you have a Solar System Effect with poison, for instance, then you can spend that for bonus dice in whatever Ability check is warranted - you might even have a Secret that allows you to turn those bonus dice into penalty dice for the victim, instead. For poisonous infusions the rules are very specific - they cause Harm and other things depending on the target's Endure check. Even then you might or might not require another check to introduce the poison into the victim's system, that's pretty completely up to the Story Guide to decide based on dramatic coordination.

The larger part of your question is still to come, though, and that's how you kill another. The poison thing is incidental here in that the same principles go for any other method of killing others as well. In TSoY you can only outright kill a character nobody cares about. If they have a name, then the best you can do is manage a "seeming death", which means that everybody in the fiction thinks that they died, but the SG actually has a pregorative to bring that character back whenever they feel like it. In the case of poison this would probably look like some sort of misdiagnosis, or perhaps the victim has himself spirited away to some secret place to recover. Or perhaps he really is dead and the SG never brings him back during the remaining campaign.

If you're killing a player character, it's likely that the player will extend the conflict once you succeed in your initial check to poison him. It is very important to state your character's goal very clearly and honestly in this sort of extended conflict, as the goal determines what the individual actions in the conflict will actually be about. If your goal is to get rid of a House heir, for instance, then the goes might concern his efforts at resisting the poison as an extended sequence of recovery scenes, for example, while the assassin himself lurks on the bottom of a well somewhere. Or if the victim's goal is to expose you as the poisoner, then the goes will probably be more immediate, with the poisoner sneaking around the house and trying to avoid capture, while the victim commands his house guard around, sweats and does whatever else a poisoned guy does when he's trying to catch an assassin. You have to frame the action at the moment in a way that makes sense for the overall narrative.