Your question is very interesting to work with.
This answer is separated from my previous one, because I feel that it is the best way to organize this information: my second answer will follow different assumptions and different formatting, as you refined your requirements.
And it is... unexpectedly long.
It would be perfect if it included elements of distance communication and compliance, such as their overlord able to make a kind of Codec call and electrocute them or similar if they start acting rebellious.
for simplicity, this can broken to three goals of quest-giver (I will call him Q in the text below):
Long-range communication. This is implementation of way Q's wish to have ability to communicate with PC and see them, and judge their actions.
Jolting. This is implementation of minor form of Q's displeasement or wrath - non-lethal but fast, intimidating and, perhaps, painful.
Retribution. This is implementation of Q's threat of unavoidable destruction of PCs when they decide to betray Q or ignore his demands.
Below I will give some tools that can work as implementation of one or a few of these goals.
But before describing these tools
I feel these things should be mentions to give a better context:
Long-range communications requirement indicate that we automatically have to deal with magical solution - no medieval technology could allow that. So Q will be using magic. No workarounds around that. -_- Eh.
Problem with scrying to summoning consequences is they may think they can win or escape from all the established counters
yep - this is important. (Hmm.. and what if they really can?) Anyways - to make threat of Retribution sink in minds of the players and PCs, Q must make a point that Retribution is unavoidable - he must demonstrate and explain and show that Retribution either
requires no spending of resources
or laughably cheap for his scale of operations
or he is using replenishable resources (spells, artifacts with infinite charges)
and even if one or two of Retribution attempts fail - he can continue attempts for a week, month. Even if his powers allow him only one attempt per one day - he could do it while PCs are sleep or are trying to sleep. Sleeping character is helpless and vulnerable to one-round coup-de-grace. It would be informative if Q shows a demonstration through Long-range communication or in a sight-range - some if his tools - perhaps not all - but a combination that is enough to execute a Retribution. This will help setting players' expectations and get them and you on the same page.
(Here is Q's motive for demonstrations: he knows that ANY unwilling low-witted pawn sooner or later will try to betray him... because HE himself would try - being in the pawn's place. And after pawn's betrayal he will have to kill it. And most probably he will succeed. But he is trying to conserve resources by intimidating them - finding and capturing some other group of mercenaries will take some time and money - not that much - but yet - spending some time on simple verbal communication is cheap. So why not? What would he lose by having a mere talk and blowing a few huts and maiming some criminals or enemy?).
It would have more weight if his Retribution plan is not rigid, but flexible and can be arranged in unpredictable manner. Uncertainty scares. It also makes players paranoid when they need to counter it. ^_^
Here is another aspect of presenting Q that could help - as I don't know his personality - but any of those two quirks can sell Retribution and idea of cooperation (at least short-termed) (these quirks shouldn't be outright declared, but could be seen through events happening around Q - background executions, or rewarding scenes):
Q pedantically (for whatever reasons - for maintaining his image or for bloody fun) destroys anybody of his hirelings or reachable enemy who committed serious crimes against him
Q reward without regrets those who carry out his will as intended (That quirk feels better. By the way it would make greedy or lawful character make decision: should they betray Q at all, alignment be damned? tension! meaningful choices!).
Now, the tools:
Those tools are spell-based. If I'd were in the Q's place, then for some stratagems, I'd find it very reasonable to craft a magic item that implements a certain spell - that would free daily spellslot to complete complex combinations of spells in a reasonable time. To make estimation of price and time to create a magic item, I used this set of rules.
SpellLevel: WizardSpells 4
Range: anywhere. Even another plane.
If Q has hair and fingernails of PC - he can scry them almost anytime even if they resist. I think he will have those.
Q can instruct PCs to fail Will Save when they feel that Q is looking for them via scrying: thus Q can contact them with guarantee. Resisting indicates betrayal and triggers Retribution.
Duration: With scrying Q has ability to see PC. 1 min./level. That's 7 min for 7th level Wizard and 9 min for 9th level Wizard. As it is - that is not enough to have a lingering presence but it is enough to make use of a Teleport stratagem (described below).
What's interesting here - if he decides to implement it as artifact though - this spell could be powerful:
Cost (infinite charges, use-activated): 56000 gp, 56 days
Minimal CharacterLevel of creator: Wizard 7
Costy in time and money - but when he has one complete - he gained a lot of power - now he has ability to contact his field-agents, or spy on his enemy. Continuously. Maybe you could nerf it so that changing target of scrying requires 1 hour, but even that allows steady visual connection that could prove themselves priceless.
That artifact version of spell could definitely serve as implementation of Long-range communication.
Spell is interesting in that you can combine it with Message. For 7th level Wizard Message has 35% of success. For 9th level Wizard message has 45% level of success. To have a chit-chat 1-4 spellspot Messages per day is too short, too expensive and unreliable. But if you make magic item with infinite charges - it will allow you a two-way communication with field agents. Not 100%-time communication (there would be some pauses) but it makes communication possible.
SpellLevel: WizardSpells 4
Makes sense only as part of combination with Scrying, and only in artifact form that allows infinite uses:
Cost (infinite charges, use-activated): 1000 gp, 1 day
Minimal CharacterLevel of creator: Wizard 1
SpellLevel: WizardSpells 5
Combined with Scrying it allows Retribution attempts. It will have 76% probability that Teleport is successful.
It could be useful to buff up field agents with reinforcements.
It could be used to send Dominated Emissary (described below) if previous Emissary meets an unexpected death.
There is a catch: teleport doesn't allow interplanary travel.
Artifact version is not costy - this is a great artifact:
Cost (infinite charges, use-activated, one charge/per day): 18000 gp, 18 days
Minimal CharacterLevel of creator: Wizard 9
Explosive Runes link
SpellLevel: WizardSpells 3
This is an old and dread legendary Stratagem of Explosive Book. It's concept works like this: you find many sheets of paper, cast Explosive Runes - as much as you can cast during a day. Then you connect pages to a book. Now it is one object. To detonate - you must cast dispell magic on Explosive Book and willingly fail your check.
As Q would want to use this through the Emissary or summoned creature - this creature must have a tool to cast dispel magic, and that tool must be intelligent item. Alternatively - creature must be magic user.
This fail will produce fireball of 6d6 x number of pages.
And here we have Summon Monster 5 with Babau demon that can cast dispel magic:
Summon Monster 5 link
SpellLevel: WizardSpells 5
From all the list of available summons we need... demon Babau.
Combined with Teleport he can work as Retribution, or to buff up field agents, or act as scout for field agents.
He is a stealth and diversions incarnate. Dropping him upon sleeping characters is coup-de-grace menace. But combined with Stratagem of Explosive Book he transform to a potent demolition expert: he has a dipell magic ability. This can allow head-on confrontation.
Q must instruct him to fail dispell check.
To give him instructions, Q must speak Abyssal language.
Summon Monster 2 link
SpellLevel: WizardSpells 2
From all the list of available summons we need... minor Air or Earth elemental.
Combined with Teleport he can work as Retribution, or to act as scout for field agents.
They can be used for Retribution - dropping upon sleeping characters they are coup-de-grace menace. If you give elemental an Explosive Book and artifact of Dispell Magic he can act as a demolition expert.
To give him instructions, Q must speak corresponding Elemental language.
SpellLevel: WizardSpells 5
You can skew rules to use this spell. You must allow caster to see through the eyes of Dominated humanoid at will, get his sensory input, and control movements. Q can switch between Dominated Emissary and Q's body by will. While controlling Emissary, Q can't cast spells, unless Dominated Emissary has artifacts.
Duration 1 day/level = 7 days for Wizard-7 and 9 days for Wizard-9 - this is about time you defined for this operation.
Humanoid that will be Dominated Emissary could be one of the loyal servants of Q, or his motive to participate in this risky operation - is the size of reward he could get. He allows Q use his body willingly.
That implements Long-range communication., and if you give him some jolting artifact - it can implement Jolting. in the same time!
You can skew rules to create 1-level spell (actually - nerfed magic missile, so it's balanced in a way)
1 damage to target (or 1d3 if you feel this proper), negates armor, stopped only by force shield.
It must be combined with Dominated Emissary stratagem and it must be an artifact form, which is cheap:
Minimal CharacterLevel of creator: Wizard 1
Cost (50 charges, spell trigger): 750 gp, 1 day
Activated without words, by gesture. Artifact could be something like a ring on a finger, or implanted inside of Emissary.
Note: I don't think that allowing jolt through scrying is a good design decision - thus I urge you not to. It will allow Q toast any PC he can scry - slow but inevitable. That would be bad-bad.
You can break rules to create a spell that is same as Dominate Person (skewed version) - but allows you rig inside undead that is under your control. Combining it with Jolt, it implements Long-range communication and Jolting.
Creating new undead is as simple as described in Animate Dead spell
Your options are: skeleton, bloody skeleton, burning skeleton, zombie, fast zombie, plague zombie.
But your Dominated Emissary must talk to PCs so I think you would need zombie, perhaps fast zombie is preferable. If zombie can't talk in your setting - there is a need to create a magic device to produce a voice from his mouth.
Zombies are boring so you can add some cosmetic touches. It could be generic part of story (body comes from faction that rivals Q's - perhaps unlucky assassin or somebody who was kidnapped by Q - and during quest somebody who knew him in life could approach with hope... 'You're alive, my friend!' then fall back in horror. Or that body could be used for infiltration to enemy base of operations.)
Or it could have some game-mechanics consequences - something like
Chest is chest - smuggling compartment in rib-cage - PCs can keep different objects there - for example loafs of dried meat.
Rat-faced - swarm of undead rats stitched to his face (8-12) like tentacles - chirping, watching, biting - in the place of eyes - used for intimidation when he grabs someone - can be a variant implementation of Jolting
Portable ram - carries a portable ram on his shoulder. Undead are strong - he will help breaking doors now.
Unnatural birth - there is another, smaller undead, hidden in the stomach of Dominated Emissary. It could be dominated too or just follow the orders of Emissary. Don't know how Q could deploy it - but there are many possibilities. I don't know where is exit is supposed to be - maybe stomach, maybe chest. It's base could be bird (bird zombies can fly, many birds can mimic sounds like crows), monkey or gnome or pixie.
Flexible joints - legend says that ninja made surgery to modify joints of their children so those could be popped in and out more easier than an average man could. They say, it was used to escape ropes and squeeze through otherwise impossible passages. Q made the same thing here.
Walking church - Q assumes that PCs are in stressed situation. This could be bad for their morale and performance. Having a bit of sympathy to them - or interest in flawless completion of mission - he offers them a religious relief. To improve their morale, he installed inside of Emissary religious icons and tools that could be taken outside and used by shaken PCs to have a prayer for all their gods. He doesn't see any flaws in his perfect logic.
Unnaturally strong fingers - good for climbing and unnaturally good for choking someone. Good for disarming too.
When one Undead Emissary is discarded - it would be more interesting when next will use different 'perk'.
Hancuffs of Contol
You can break rules to create artifact that allows telepathic link to a wearer of handcuffs, see through his eyes at will and to jolt him on will. Master is informed when wearer is trying to open handcuffs. (To give you a perspective - permanent telepathy allowed from 13th level)
Why not use familiar? They give their special advantages only when in 1 mile from their master.
What is Q?
Does Q have to be magic user of 9th level?
Nope. Maybe he signed a pact and those artifacts are given by his patron?
There could be a situation where a wizard of past created arcane devices (for the purpose of controlling his field agents) he died, and now his treasures are in the hands of Q. Maybe he could be a favorite student of old wizard?
Or he is just a bandit lord who got a hold on artifacts of dead wizard?
Could it be that one of artifacts is a stationary magic resource created by nature - not unlike a vein of ore?
What is his goal? Power, order, chaos? Or maybe he is a well-intentioned extremist gone too far?
But I still see him as Wizard 7th or Wizard 9th + organized minions.