Hammer of Math: August 24, 2025 - Dark Pacts and You

In Hammer of Math, we look at statistics and math-related problems in Warhammer 40k, talking about how they affect games and how you can use them to your advantage. Today we’re talking about Dark Pacts across various detachments and armies.

We've talked about Dark Pacts before but it's worth revisiting the rules and statistics around them now that there are multiple ways to do them across different armies, each with different effects.

Dark Pacts in Codex: Chaos Space Marines

Dark Pacts is the army rule for the Chaos Space Marines. If your Army Faction is Heretic Astartes, each time a unit with this ability is selected to shoot or fight, it can make a Dark Pact. If it does, it must first take a Leadership test before any effects of the Dark Pact are resolved; if that test is failed, that unit suffers D3 mortal wounds. Then, select one of the following abilities for that unit’s weapons to gain until the end of the phase:

[LETHAL HITS]
[SUSTAINED HITS 1]

Pact rolls have to be taken before any bonuses are applied, and it's entirely possible to lose a model or blow up before you get to the shooting/fighting. Additionally, some effects – such as the Mark of Chaos bonuses in Pactbound Zealots – only trigger if you pass your test. Most Chaos Space Marines have a Leadership Characteristic of 6+, so our odds of passing that test are a little better than 72%. If we have an Icon to re-roll the test, those odds jump to 92%.

That said, there are a few other places you'll see different values for these - with Abaddon or a Dark Apostle your Ld value improves to 5+, giving you 83.3% base odds of passing your Pact, or 97.2% if you have the ability to re-roll the test.

On the other side of things, weaker-minded units like Chaos Cultists may only have a Leadership value of 7+, or 58.3%, and if that drops to 8+ due to modifiers you'll only be passing that test 41.7% of the time.

The Detachments

There are four key Chaos Space Marines detachments that affect or make use of Dark Pacts:

Pactbound Zealots. Still the gold standard, if you're playing this detachment all of your units have a mark keyword that can give them critical hits on a 5+ depending on whether they are shooting or fighting and whether you chose Lethal Hits or Sustained hits. Most of the time you'll use this to generate Sustained Hits with the marks of Nurgle and Slaanesh, or just re-roll hit rolls of 1 with the Mark of Chaos Undivided. Note that you only get this benefit if you actually pass the Leadership test.
Chaos Cult. This Detachment gives you an entirely new type of Pact you can do when one of your units makes a Normal or Advance move or declares a charge. Desperate Pacts follow the same rules as Dark Pacts, but when you make one (you don't have to pass the test), you get +2" to your unit's Move characteristic and +2 to their charge rolls for the rest of the phase.
Soulforged Warpack. Daemon Vehicles in this Detachment can opt to attempt their pacts at -1 Ld in order to get +1 to wound with ranged weapons and +2 attacks with melee weapons for a phase. They can further improve this with the Tempting Addendum Enhancement, which gives them re-rolls to hit when they pact if they take an extra mortal wound when failing (bumping the average failure cost up to 3 mortal wounds from 2).
Cabal of Chaos. When you attempt a Dark Pact, you can pick an extra ability for your unit to gain until the end of the phase, one of which keys off being within 9" of a friendly Heretic Astartes Psyker (+1 strength in shooting) and the other off being within 9" of a friendly Daemon Prince (+1 AP in melee).

Most of the time you're working with these Detachments you'll be looking at need to roll either a 6+ (72%) or 7+ (58%) to succeed, with more of those 7+ results showing up in Chaos Cult and Soulforged Warpack. Note that any time you roll a D3 to determine your mortal wounds, the average number you can expect to take is 2. This can be helpful when you're trying to figure out whether it's worth chancing it on a wounded single model unit before you attempt a Dark Pact. Generally speaking the downside of losing a whole model is typically not worth the upside you get from a pact when it comes to smaller, more elite units - having a second Obliterator to shoot is worth much more than having Sustained Hits 1 on the surviving model in the unit.

The ongoing debate around Chaos Space Marine Pacts is always Sustained Hits vs. Lethal Hits. And generally the answer is that you want Lethal Hits if you're rolling a 5+ to wound and Sustained Hits all the rest of the time, with the break-even mark being a 4+ to wound and at that point Sustained being better because it has more upside. On that note, if you're running Cabal, you're likely to find that there's a non-combo with +1 strength on your shooting attacks and Lethal Hits, so you'll most likely want to opt for Sustained Hits there if it gets you over to the 4+ breakpoint.

Credit: Robert "TheChirurgeon" Jones

Chaos Knights

There are two places Chaos Knights can make Dark Pacts, or what are functionally indistinguishable from Dark Pacts:

the Malefic Surge rule in the Infernal Lance Detachment lets Chaos Knights take a Leadership test to become Empowered, taking D3 mortal wounds if they fail but still giving them the buff. From there they can either pick Unholy Hunger (+3" Movement), Diabolic Power (Lethal or Sustained Hits when they shoot or fight for a phase), or Unnatural Fortitude, giving them a 5+ invulnerable save and a 6+ feel no pain for a phase.
The Dark Sacrifice rule in the Iconoclast Fiefdom Detachment lets Chaos Knights throw away nearby Damned models (within 6") to essentially gain the benefits of a Dark Pact, either Lethal Hits or Sustained Hits 1. Pass the test and the nearby unit loses D3 models (average 2). Fail and they lose D3+3 (average 5). Note that this incurs a loss of models and not mortal wounds, and so you wont' stop it with Feel No Pain. Also, the Leadership test is on your DAMNED unit, so usually 7+ unless they have a Cultist Firebrand with them.

Here the big rub is that smaller Chaos Knights have worse Leaderhsip values - War Dogs only have Ld 7+, and that means your odds will generally be lower with them than you might be used to if you've been playing Chaos Space Marines more often. Otherwise, the same logic applies here as did to Chaos Space Marines when it comes to Lethal vs. Sustained Hits, i.e.:

Lethal Hits vs Sustained Hits

Regardless of the hit roll, the equilibrium point between Lethal Hits and Sustained hits is a wound roll of 4+. Sustained Hits 1 is more effective against targets you wound on 2+ or 3+ while Lethal Hits is more effective against wound targets of 5+ or higher. This is unaffected by changing the Critical Hit target. For Sustained Hits 2 (or Sustained Hits D3), the equilibrium point shifts to a wound roll of 5+, meaning that Lethal Hits is only better against targets with a wound target of 6+.

If you're combining re-rolls with your critical effects, remember that the hit roll doesn't actually affect which one you should choose, and so the ability to re-roll misses or 1s doesn't influence which option you should take. That said, you should only fish for crits when you're hitting on a 2+ or when the wound roll is 6+. Otherwise you're better off just re-rolling misses.

Final Thoughts

It had been a while since we updated our thoughts on Dark Pacts but with the relatively recent Codex: Chaos Knights it was worth revisiting both armies and their ability to push their luck for better outcomes. In most cases it'll be right to push your luck, though you will come across more than a few scenarios where it doesn't make sense in games, because the cost of losing a model is too high. At least, that's what you'll tell yourself, then realize that it would be cowardly not to invoke the dark gods right before you watch Abaddon fail his 5+, take three mortal wounds, and die without ever getting to attack.

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