Games of BOS are played by alternating between players, each taking a turn. A turn is split up into 4 phases.
During your startup phase, the following steps occur in this order:
Ready your face-down wizards and acted cards one at a time. (If something readies during the startup phase and then acts due to an ability, it does not get readied again)
Double your vim.
If needed, decay. Decay is a mechanic that resolves ties and ensures games don't last forever. To decay, you halve your life and vim (rounded down) for each turn you have started after your 10th. For example, if it is your 11th turn, you halve your life and vim once. If it is your 13th turn, you halve your life and vim three times. (For those worried, games of BOS typically last around 4-8 turns for each player)
During your summoning phase, you may conduct summons using your wizards. To do this, follow these steps, as many times as you'd like:
Select the summon you want to play from your castle.
Check the summon's condition to see if it can be played.
Turn a wizard face-down, who does not already have a summon in front of it and who matches the summon's condition.
Place the summon onto the battlefield in front of that wizard, ready.
Use abilities that occur when that summon is summoned, if any.
During your main phase, you may perform any of these actions as often as you want, in any order, in any combination:
Use abilities that occur during the main phase
Summon your boss
During your ending phase, nothing special happens. Some abilities occur during this phase, and it is meant as a transitioning phase. Next turn!
Phases have three states which they can be in: Start, middle, or end. Some abilities specifically occur during these states, such as Moonlight Flower. Any special actions specific to a phase occurs during the middle state, such as readying and doubling vim in the startup phase. This means that any ability that occurs at the start of a phase occurs before any special actions are taken during that phase.
Playing with more than 2 players
BOS does support games that have more than 2 players! This can be done in teams or in a free-for-all. The recommended ways to play are:
Teams: Players team up, with each player having their own life and vim counter. Teammates are not counted as opponents.
Free-for-all: Each player gets their own life and vim total, and must be the last person standing to win. Every other player is counted as an opponent.
2 vs 2 combined: Players form a battlefield with 6 wizards, 3 from each deck. Teammates take their turns at the same time. Each team gets one life and vim total, and now may have 2 bosses.
Raid boss: 1 player versus all. First, the raid boss begins the match, and they get the advantage of being able to attack a player directly even if they have not started a turn. Then, one at a time, each player afterwards takes a turn. A defeated player does not put away their cards, but instead keeps them in the exact way they were left before they were defeated. A defeated player skips their turn and cannot perform any actions. A player has an option of skipping their summoning and main phase in order to revive another player, bringing them back to 30 life and 1 vim, and putting their boss back in its starting position if it was already been defeated. The rest of the battlefield is not effected.