Hammer of Math: Basics of Horus Heresy

This week’s Hammer of Math takes a look at the rules behind the newly released edition of the Horus Heresy, and how things might be a bit different for a player used to 9th Edition.

I’ve been playing Warhammer 40k on and off since 2nd edition. One of my larger gaps was between 6th edition (I bought the starter box and never played a game) and 8th edition, which I picked up at release and have been deeply involved in the wargaming community ever since. What this means is that I missed the entirety of 7th edition along with the release of Horus Heresy. By the time I got back into the hobby I always regarded HH as this interesting niche full of incredibly talented painters and gorgeous models. With Games Workshop releasing a new edition and transitioning a lot of the product line to plastic I look forward to seeing even more amazing work, and the feedback from several of my friends is that it’s an amazing game to play. Maybe one day I’ll reduce my backlog enough to consider getting into it, but I suspect there are many 40k players out there who are Heresy-curious and the goal of this article is to look at some of the probabilities behind the Horus Heresy and how they’re going to be different from 40k.

Contemptor. Credit: Rockfish
Contemptor. Credit: Rockfish

Hitting Heresy

Horus Heresy uses a legacy BS/WS system which differs from the check-focused attributes that 8th and 9th edition have used. Instead you reference a chart to find out what your target number is. For the most part the probability to hit is simply 7 – BS, but high levels include an innate re-roll with a different target number on the re-roll. For example a BS of 7 will hit on a 2+, and if the hit roll is a 1 then the player will re-roll and can hit on a 5+. In the event of a special rule that provides a re-roll then the chance of hitting with the re-roll is the same as the first roll.

Hitting in melee is completely different from 40k, with models comparing WS to get a final probability. The chart is actually identical to how wound rolls work in 9th edition. The basic hit roll is a 6+. It becomes a 5+ if the defender WS is greater than the attacker WS but not twice as high. It becomes a 4+ is the WS of the two fighter is equal, a 3+ if the attacker WS is higher than the defender skill, and a 2+ if the attacker WS is equal to or greater than twice the defender WS.

Salamanders Deredeo. Credit: Rockfish
Salamanders Deredeo. Credit: Rockfish

Horus Heresy also has Blast weapons which require actual Blast markers, with circles ranging from 3″ all the way up to 10″ in diameter. There is also a flamer template that’s 8″ long, an ‘Apocalyptic Barrage’ marker which is a clover shaped template of 5″ diameter circles, and a ‘Hellstorm’ template about 16″ long. The bigger templates are from when GW released an alternative Apocalypse rules set which was released in 2007 and was a lot of fun if you had tons of friends and most of a day to throw dice. Blast weapons require the player to roll 2d6 and the Scatter die (a six-sided die with a Hit symbol on one face and arrows on every other one). If a Hit is rolled then the marker doesn’t move, otherwise the weapon scatters in the direction of the arrow for the 2d6 distance in inches. Assuming the Blast template is centered on the target this means larger bases have a significantly higher chance of hitting. At some point in the future I plan on doing probability maps for the chance that a given location will be hit in an area around a target.

Wounding Heresy

Horus Heresy uses a chart to determine the probability of wounding a target based on the relationship between the Strength characteristic of the incoming attack and the Toughness characteristic of the target. While it appears similar to the wound rolls of 9th edition 40k, there are subtle differences which players may want to take note. First, there is a toughness level beyond which an attack has no hope of wounding. Second, as the Strength characteristic of the attack increases the probability of wounding increases dramatically. The chart below shows the wounding requirements as well as combinations where the probability is worse (italicized purple) or better (bolded orange) when compared to 40k. Attacks in Horus Heresy also receive the Instant Death rule, which states that if the Strength characteristic of the attack is at least double the Toughness characteristic of the target then if the model suffers an unsaved wound it is automatically removed as a casualty.

Saving Heresy

40k players used to the gradual decline of armour saves should note that Horus Heresy uses an armour penetration system; if the AP characteristic of the attack is equal to or lower than the armour save of the target then the target does not get a save. Models with an invulnerable save can still make a Save roll regardless of the AP of the attack, and cover provides a 6+ invulnerable save to units. The end result is that at a certain threshold the lethality of an attack increases dramatically, creating a step function in terms of probability of wounding the target.

Imperial Fists Spartan. Credit: Jack Hunter

Driving Miss Heresy

The final major difference we’ll cover in this article is vehicles. In 9th edition all models, whether vehicle, organic, or something else, uses the same stat line. In Horus Heresy vehicles have facings and Armour Values which generally range from 10 to 14. They also have Hull Points instead of wounds. For example, a Land Raider Proteus has 5 Hull Points and 14 armour on every facing. Compare this to T8 with 16 wounds and a 2+ save that ignores the first point of AP. In lieu of wounding and saving, when an attack hits a vehicle the player rolls a D6 and adds the Strength characteristic of the attacking weapon. If the total is less than the Armour Value then the hit does nothing. If it equals the Armour Value then the attack has a Glancing Hit that removes a Hull Point. If it exceeds the Armour Value then the attack is a Penetrating Hit that removes a Hull Point and forces the player to roll on the Vehicle Damage table. AP 2 and AP 1 weapons get +1 and +2 when rolling on the table. The vehicle is Wrecked when it loses all of its Hull Points or a 7+ is rolled on the Vehicle Damage Table and the tank explodes. The Vehicle Damage Table primarily consists of degrading effects, such as Crew Shaken which forces the vehicle to only fire Snap Shots (attacks which only hit on a 6+) for a turn or various weapons being destroyed. In terms of probability this means that tanks are heavily gated against minor weapons. An AV 14 Land Raider will simply not care about any attack unless it’s S8 or higher, and a S9 or S10 attack is required to achieve a Penetrating Hit. AP 1 and AP 2 weapons are particularly nasty, with an AP 1 weapon having a 33% chance to cause the target to Explode regardless of how many Hull Points it has left.

Wrapping Up

Heresy looks to be a lot of fun. I vaguely recall a lot of similar rules from when I played earlier editions, and it seems like Games Workshop has taken the time to tweak some things. If you already play 40k and are interested in seeing what happens when a giant pile of power armour gets to play with Dark Age weaponry then it might be worth checking out! Just remember that the Horus Heresy rules are going to be significantly different than 40k, and don’t be surprised when things don’t go as expected.

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