Reading the Spirit Island Invader Deck: Odds and Counting

At Level 1 you learned to read the two face-up Invader cards, the ones about to Build and Ravage. That shows you two turns into the future. This lesson goes further. The deck itself is not a mystery. You know every card that could be in it, and each card you see tells you more about the ones you have not.

What is in the box

The core game has 15 Invader cards:

• Stage I: 4 cards, one each for Jungle, Mountain, Sands, and Wetland.
• Stage II: 5 cards, one for each of the four terrains plus one for Coastal lands.
• Stage III: 6 cards, one for every pair of terrains: Jungle and Mountain, Jungle and Sands, Jungle and Wetland, Mountain and Sands, Mountain and Wetland, Sands and Wetland.

A game without an adversary uses 3 Stage I cards, 4 Stage II, and 5 Stage III. The rulebook notes you can simply remove one card from each stage at random. That means exactly one card is missing from each stage, and you never know which until you work it out.

Stage I: one terrain sits out

Three of the four single terrains will be Explored, one per turn: during setup, on turn 1, and on turn 2. So each terrain has a 3 in 4 chance of appearing in Stage I.

After you see the first two Stage I cards, the third is one of the two terrains you have not seen, a 50% chance each. So from turn 1 on, you know the turn 2 Explore will hit one of two terrains, and that tells you where Builds are likely on turn 3 and Ravages on turn 4.

Stage II: the Coastal card

Stage II brings the first card that is not a terrain. The Coastal card Explores in every Coastal land on the Island, of every terrain, which usually means more lands than a one-terrain card. It has a 4 in 5 chance of being in the deck.

Most Stage II cards carry the Escalation flag that triggers an adversary’s Escalation. The Coastal card is the one without it, so with an adversary, you will see four Escalations if the Coastal card is the one left out, and three otherwise.

Stage III: the missing pair

Stage III is where counting pays off most. Each terrain appears in exactly 3 of the 6 pairs. One pair is left out, so:

• Two terrains appear in all 3 of their pairs.
• The other two appear in only 2.

At the start of Stage III, any given terrain has a 50% chance that the next card includes it, because 3 of the 6 possible pairs do.

As you see cards, update the count. After you have seen some Stage III cards, the next card is equally likely to be any pair you have not seen yet, because one of them is out of the game. With 4 pairs unseen, each has a 1 in 4 chance. With 2 unseen, it is a coin flip.

To find a terrain’s chance, count how many of the possible pairs include it and divide by the number possible.

When each card matters

Number the cards 1 to 12 in reveal order. Card 1 is revealed during setup, and card N is revealed on turn N minus 1. Each card Builds the turn after it is revealed and Ravages the turn after that.

The last card is revealed on turn 11 and Builds on turn 12. On turn 12 the Explore finds no card, and you lose before it can ever Ravage. So the twelfth card’s Ravage never happens; your win must come by then.

Using the odds

• Placing Presence: when the next Explore is a coin flip between two terrains, place new Presence in a third. Lands Explored now Ravage two turns later.
• Saving a Power: if a Slow Power would be perfect against one terrain, and that terrain is likely next, hold it a turn.
• Late Stage III: when only two pairs remain and they share a terrain, you know that terrain will be Explored. Prepare it.

Adversaries change the deck

Some adversaries rebuild the deck. Brandenburg-Prussia moves a Stage III card earlier and removes cards at higher levels, and Sweden’s level 4 Accelerates the deck during setup. The counting method still works. Start from what was removed and what order the stages are in, then track what you see. The Level 4 lessons give each adversary’s deck in full.

Counting is a habit, not a calculation. After a few games you will know at a glance which pair is missing, the same way you learned to read the two face-up cards at Level 1.