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Rad counters are a kind of counter a player can have (see rule 122, “Counters”). There is an inherent triggered ability associated with rad counters. This ability has no source and is controlled by the active player. This is an exception to rule 113.8. The full text of this ability is “At the beginning of each player’s precombat main phase, if that player has one or more rad counters, that player mills a number of cards equal to the number of rad counters they have. For each nonland card milled this way, that player loses 1 life and removes one rad counter from themselves.”