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Initiative determines the order of turns during combat. When combat starts, every participant makes a Dexterity check to determine their place in the initiative order. The GM makes one roll for an entire group of identical creatures, so each member of the group acts at the same time.
The GM ranks the combatants in order from the one with the highest Dexterity check total to the one with the lowest. This is the order (called the initiative order) in which they act during each round. The initiative order remains the same from round to round.
If a tie occurs, the GM decides the order among tied GM-controlled creatures, and the players decide the order among their tied characters. The GM can decide the order if the tie is between a monster and a player character. Optionally, the GM can have the tied characters and monsters each roll a d20 to determine the order, highest roll going first.