Spoiler: Main idea
Show
Did you say "
Unstoppable Force"?
You'd find it nigh-impossible to evade or escape this round's villain.
Oh yeah! You can't escape a villain if you don't know it exists. It hides right here in your own shadow!
And while it's here it could make... nasty!
Spoiler: Early CR (4)
Show
I think 3rd CR is out of game because this umbral spy was created with extra HD.
Bringer starts as an almost regular advanced Umbral Spy. But it has human-high intelligence and slightly better another stats. Also it has Sudden Ability Focus instead of Improved Initiative. It works perfectly with one per day save against Bringer's Shadow Bond.
I think at this CR PCs should meet Bringer while it still is under the control of some friendly NPC. Maybe even get some information for the PCs. But after that its master should be assassinated and Bringer's control talisman destroyed. Maybe it even should be revenge of the PCs’ enemy that Bringer was spying on. Some demon worshiper, I guess. That's why Bringer turned to a warlock.
Spoiler: Low-mid CR (5-10)
Show
Bringer is a warlock now. It's good for its goals - to hide and to harm. It selects PCs as its main targets. It hides in the shadow of the party's main fighter or rogue - somebody with weak will - it has enough time to figure it out. It establishes its Shadow Bond in the darkness of night, when it's more easy for it to hide and when its targets tend to sleep and even if it fails to bond or bond breaks - nobody will notice. Cloak of Deception also helps.
What harm does it make?
It could Summon Swarms. At the most inopportune moments. When PCs sleep, or bathe, or... I think you could imagine more inopportune moments.
Also it has Drain Incarnum. It's great if one or more PCs have connection with Incarnum, but if not - it's wisdom damage! Free! Every turn. If Bringer wanted it could put every or some PCs into a coma in a few minutes. Perhaps without disturbing their dreams.
Steal Incarnum isn't so good. It works only against targets with essentia and therefore is limited.
And of course Darkness in case it is necessary to act on the light.
Plus one Eldritch blast at inconvenient time aimed from one of PC's place could disrupt any negotiations with unfriendly or even friendly creatures.
Spoiler: High-mid CR (11-15)
Show
Highly likely PCs have True Seeing at these levels, but True Seeing doesn't help against hiding or being in solid objects - Bringer says "thanks" it's incorporeal. Yes, PCs probably already know they have a fan, but even then Bringer could make some nasty! First of all, PCs need to have continuous True Seeing and high Spot check. Unusual senses, by the way, don't help very much. Tremorsense, scent, blindsight don't work against incorporeal targets. It means - expenses. Second - same story about mind protection. All day and all night long for every PC. And Bringer still could use Drain Incarnum and Summon Swarm. And one of new invocations - Charm. Charming some passerby and persuading him to attack PCs, or short-changing them in a shop, or just spit in their soup. And this is Charm Monster and it works against almost anybody, so every night in the wild guarantees an animal (or dragon?) attack or multiple.
Also Bringer could be careful, didn't attack PCs every night or even break the bond for day, week, or even months. Or PCs could just not have access to True Seeing. And the party could still not notice Bringer. This is the better option. If so Bringer has some additional nasty possibilities.
First - Charming one of the PCs and persuading him to do something stupid.
Second - Bringer has an Crawling Eye invocation. It could be used to bond another PC, but the main idea is different. There are no limitations on duration and distance in this invocation and Bringer can see through the eye. And it has Thought Link ability. With Crawling Eye Bringer can see some really disgusting or gruesome things and share them with PCs, or it could spy party members' families and share some dirty secrets or just not unpleasant scenes - it's could be not best experience for somebody to see his wife picking her nose, for example. If one party member has any secrets from other members... Well, he has no secrets anymore. Best time for such revelations is when PCs are going to sleep of course!
Spoiler: Latest CR (16-20)
Show
If Bringer still is not discovered, Devil's Whispers is great for it! Now Bringer can bring discord truly! Attack the ally? Or just insult him (with a reason of course!)? Or maybe insult high-ranking officials? So many possibilities for nastiness!
If Bringer wants to confront PCs (or provoke somebody to attack PCs) Chilling Tentacles helps it.
Incarnum Shroud isn't the best invocation. But 20% miss chance could be useful in some circumstances and 1 point of essentia isn't superfluous.
I should mention Harvester of Souls feat. Any PC or other creature putted in a coma by Bringer is in a big danger now! Well, it always was dangerous, but from this moment on... Even resurrection isn't option.
And our star - Souleater Incarnate! While PCs fight with somebody (after Bringer's provocation) Bringer in the shadow one of them is reaping the fruits. More dead enemies - more temp hp, bonus to AB and saves. If some of the PCs die - good luck to resurrect him, Bringer even doesn't need to make coup de grace. Also plus 3 essentia to improve Drain Incarnum's DC.