You don’t pull fast ones about the mission goals, the mission rewards, information gained from intel, and so on. You don’t lie about things they earn, or will earn, or things coming from resources they spend. But that doesn’t mean you can’t mislead or confuse characters during a mission with false information, mistaken beliefs, or deceptive tactics, or that you can’t have mystery in the game.
There are multiple systems in the game to mitigate all of this if the players ever feel like any of it is a raw deal, such as flashbacks and resistance rolls. “But luckily, we knew this was an ambush and prepared for it this way…” or “Nope. I wasn’t fooled. Here’s what happens instead…”
Devil’s Bargains are another good one: “OK, for that die: someone in your squad is Hexed, but you don’t know who. They’ll turn on you at some point. Anyone up for it, or should it be an NPC squaddie?” or so on. This creates some psychological tension, but everyone at the table is in on it.
I bring this up because with Render and Blighter, there are things the players don’t know, which remain either mysterious, or aren’t simply and outright revealed unless the players pursue the answers: “Are there squads of Gaunt over that hill?” “How exactly does the goop Blighter uses turn people into undead?” “How do elites actually control their undead minions?” “We know the danger here is threat 3…but what is it?” Etc. So “don’t lie” isn’t a matter of simply telling the players everything.
As you note, one of the GM principles is to telegraph danger. But not necessarily reveal it outright.
In the case of Breaker, these can be things like voices in the fog that lead characters astray, flickering will-o-wisps guiding the squad into sucking bogs, people or animals that aren’t who or what they appear to be, cries for help that turn out to be ambushes, haunted villages where the characters feel ghostly hands touching them, etc.
But however it is approached, the point of the “don’t lie” principle is actually that you don’t screw them out of standing victories: if they’ve spent the actions or stress or resources to find something out, they’ve found it out. If they’ve fought and been injured, spent stress or used intel, or made action rolls to overcome something, they overcame it. It wasn’t an illusion. (Or at least it wasn’t an illusion that undid forward progress. “You’ve broken free of the Shadow Witch’s illusions. I’m going to tick her clock: you’re wearing her down!”)
Remember, you’re essentially writing a book with everyone else at the table, so you can also just lay all the cards out: “They’re lying to you. Want to make a resistance roll to catch them out, or are you going to haplessly eat the poisoned food?” (Of course, your players have to be willing to have their characters react as if they don’t know what’s really going on and have them walk blithely into jeopardy.)
Consider also how powerful undead and lieutenants can act before the players: “Elia possesses you. Do you want to resist? How should we handle this? You play ‘as’ her? Or pick another squaddie? Maybe someone else notices something’s wrong at some point, but everything looks kosher for the moment?”
In this way, everyone at the table knows, even if their characters don’t. And then you play out or quickly narrate some fiction of how, exactly, the squad figures or finds out, how it gets them in trouble, because all of you are in on this story.
So there are multiple ways to handle things, without having to reveal everything up front.
OK, so HEXES! Do all hexes need to be carved into their victim’s flesh to work, like with Hexed? Can they be drawn in the air? Do they need hair and toenail clippings from a victim? Do the witches need to carve the hexes into their own skin? This is all left pretty open. What we do know that Shadow Witches can only empower one hex at a time.
With all that answered, the description in the book does give some ideas for when the squad is fighting things that can cast hexes. And they’re attacks just like any other, just called “hexes” instead of “zombie bites”: Your arms are suddenly bound! Your food has been poisoned! You become confused and start seeing things! The trees start grabbing you! And then you figure out if it needs a pre-resist to avoid harm and corruption, or if it is a consequence from action rolls. (Just as you do with more “traditional” attacks.)