Blackstone Fortress Probability Guide

Blackstone Fortress is a popular tabletop strategy/combat game set in the Warhammer 40K universe. The two charts below will help players weigh different strategies by estimating probabilities of success or failure.

BSFProbFigure1
Table 1 – Success probabilities for standard rolls, with defense

Understanding the table

Table 1 shows the success probabilities for common attack and defense die. Success rates are broken down in terms of normal hits and critical hits. For example, let’s say that you want to know the probability that an uninspired Janus Drake will score a critical success against a Traitor Guardsman using his pistol and rapier attack at a range of 1 hex. Janus’ character card shows that he will use a purple d8 die for this attack, so will use the 2nd row of the table. Traitor Guardsmen have no defense roll, so we will use the 1st column of the table. The entry in the 2nd row, 1st column shows that the attack will have a 25% chance of scoring a regular hit and a 25% chance of scoring a critical hit.

Let’s now reverse the roles. What is the probability that a Traitor Guardsman armed with a flamethrower at a range of 2 hexes will score a hit on Janus? The Traitor Guardsman’s character card shows that he will use the d12 attack die, so we will use the 3rd row of the table. Janus’ character card shows that he defends with a d8 die, so we will use the 3rd column of the table. The entry for the 3rd row, 3rd column shows that the attack will have a 25% chance of scoring a regular hit and a 17% chance of scoring a critical hit.

Using the table

Of course, bare success and failure probabilities are not very interesting. How can we apply this information?

The most obvious use is in planning attacks; these tables will help you decide how many dice you’ll need to allocate to eliminating various enemy groups with various explorers. For example, we need to take down two Ur Ghouls with Amallyn. How many dice will she need, on average? Amallyn’s sniper rifle uses two d8s. According to the table, one undefended attack has a 31% chance of achieving a normal success and a 44% chance of achieving a critical  success. Another way to think about this is that a single attack will produce 0.31 normal hits (delivering 1 wound) and 0.44 critical hits (delivering 3 wounds). What if we do two attacks? On average, they will produce 0.31*2 = 0.62 normal hits and 0.44*3 = 0.88 critical hits. In the same way, three attacks will produce 0.31*3 = 0.93 normal hits and 0.44*3 = 1.32 critical hits, etc… Since each Ur Ghoul can take three wounds, we’ll need to set aside five dice, which will -on average- produce 1.55 normal hits and 2.20 critical hits, to ensure that Amallyn kills them.

Of course, we could get lucky and kill them with only three attacks. Or we could get unlucky and need six or seven attacks to kill them. But the probability table gives us a way to estimate approximately how many dice we should set aside for each hero to meet their combat goals.

Unusual rolls

The first table includes only the most common dice rolls. However, other dice rolls are possible. For example, if Amallyn’s uses her sniper rifle with a 4+ die, she gets use two d8’s and reroll failed attacks. Alternative, some items allow you to reroll failed attacks with d6’s or d8’s. These combinations aren’t shown in Table 1. However, we can consult Table 2 to approximate the strength of these unusual rolls.

BSFProbFigure2
Table 2 – Success probabilities for all rolls, no defense

Table 2 shows the each dice combination from weakest to strongest. To obtain probabilities for an unusual combination, like a rerolled d6, consult the table and determine which ‘standard’ rolls this non-standard roll most resembles. In this particular case, a rerolled d6 is roughly halfway between a d8 and a d12. Then return to Table 1. If you want to know the success probability of a rerolled d6 against various defense rolls, scan down row 2 (corresponding to a d8) and row 3 (corresponding to a d12). The effects of a rerolled d6 will be roughly halfway between these two standard rolls.

For example, if we’re facing a d6 defense roll, a d8 attack roll yields success probabilities of 21% and 17%, while a d12 attack roll yields success probabilities of 28% and 22%. A rerolled d6 attack will then yield success probabilities that are roughly halfway between the two: 24.5% and 19.5%.

I hope this information is useful for mapping out BSF strategies. Stay tuned for my upcoming BSF strategy guide!

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