Wow. I said I'd never post about the game mechanics again due to all the rubbish I get for my efforts, but wow. I mean just wow. Even I find this thread confusing, since it's largely full of conflicting, yet bizarrely still correct observations. And just for good measure, even a potentially misleading contribution from the usually reliable Edi (although that's likely more down to unfortunate wording than anything else, since "script" has it's own meaning in programming terms, but a very precise meaning in Dominions terms).
Plus of course the usual load of crap from Gandalf talking about using Pan's as Charm assassins, which is something that hasn't even got the remotest of practical uses of any sort in a MP game, outside of the most bizarre settings ever played (such as one where any commander unit with a MR over 10, and buying any PD, and having more than one commander in a province, are all forbidden). And I doubt it even has a use in SP games, which is amazing since everything works in SP games, but I'd put a small wager on this being one of the rare things that doesn't work.
So I'm posting to clear things up, as a lot of what I've seen is actually making me ill
Anyway, to the matter at hand.
Assassinations.
First off there is only one script that a player can set for a commander to follow, and that is the one everyone knows about. Edi made reference to a "Regular Combat" script, and an "Assassination" script, but I'm assuming he is talking about scripts in the game code here, and not in-game commander scripts. Although a commander can revert to default / unscripted orders in very specific circumstances, which I will come onto next. (ie. the "set battle orders" you see for unscripted commanders)
Secondly, there are two different types of assassinations, which the game engine treats slightly differently (the game engine treating them differently is not something I have seen posted anywhere before, so I'm not surprised if it is fairly unknown). There is one type that happens in the Magic Phase via spells, and one that happens in the Assassination Phase via the "Assassinate Enemy Commander" order. Which is an order that can be given to commanders with the assassin tag when they are in a province (usually hidden) that is not controlled by the player issuing the assassinate order.
Note that there does not have to be an enemy commander in the province for the "Assassinate Enemy Commander" command to appear. Also please note that control of the fort does not mean control of the province. Plus you can not issue assassinate orders when you are sieging enemy forts, as you are the player who controls the province in that case, and the "Assassinate Enemy Commander" order does not appear if you control the province. But you can issue assassinate orders from inside a sieged fort (as you don't control the province then), and is one of the rare (only?) instances of being able to use the Black Heart on a non-stealthy commander in a practical way.
The main (only?) difference between the two types of assassinations, besides when they occur in the turn resolution sequence, is in how the game engine deals with any "retreat" orders that have been scripted. As in Magic Phase assassinations, which occur via well known spells like Earth Attack, Manifestation, and Infernal Disease, retreat orders are carried out whenever they occur in the commander script. But during assassinations that occur in the Assassination Phase, any retreat orders that have been scripted on the rightplayer's commander (ie. the defender) are ignored, and instead replaced with default orders when they appear in the script (ie. "Retreat" will see the commander instantly revert to default script. While "Hold x 5, Retreat" will see the commander holding for 5 turns, and then reverting to default script). Although interestingly, the leftplayer (ie. the assassin) will retreat if scripted to do so.
But to all intents and purposes, and most importantly for practical usage in MP games, there is only one type of assassination, and that is the Magic Phase one. As scripting retreat on any important commander runs the huge risk of instant death should the commander be hit with an assassination spell. And since Magic Phase assassinations make up around ~90+ of all the assassinations that occur in a regular MP game, it is only the mechanics for Magic Phase assassination that really matters. I wouldn't imagine it being too difficult for IllWinter to make commanders ignore retreat orders during Magic Phase assassinations, in the same way they do during assassinations that happen in the Assassination Phase. But at time of writing (patch 3.26) they don't, so as I said, for practical MP uses, it's better to think along the usual lines of Retreat orders = Instant death during assassinations.
And regarding using assassins to capture provinces. Assassins can not be used to conquer enemy PD (at least not by using the assassinate command), as the commander units (and indeed all PD units) that you get with PD are not "real" units (ie. they don't exist outside of the battle), so they can not be targeted by assassins. And the art of capturing Indy provinces using assassins is to know how many commander units are usually generated by the different pop-types. P3D has listed some of them, but the best way to get a complete list, like the way to get a lot of answers to the game, is simply to run some test games and make note of what you find and observe.
Finally, and as per my usual MO, attached is a test game to back-up what I've said above. (End the Turn to process the results. The game uses no mods, and the Silent Seas map, which you should get with a base install of Dominions)
The test game has two assassinations happening in Tengpoche #31. Both involve Indy commanders being assassinated, with both Indy commanders being scripted to 1st round "Retreat" orders. One assassination will occur in the Magic Phase via the Infernal Disease spell, and the other will occur via a regular assassination in the "Assassination Phase".
The result should be that the commander that is targeted by the Infernal Disease will retreat as he is scripted to do so, but the other one will not retreat as scripted, and will instead revert to default orders. (which is hopefully what I've managed to ramble on about in this message)
Re: Taking over territories with Assassins...
Linear Mode
