Screeching Scorchbeast

Rad counters are a kind of counter that a player may have. They’re not associated with any specific permanents.

“Do this only once each turn” lets you choose whether or not to create the appropriate number of 2/2 black Zombie Mutant creature tokens as the triggered ability resolves. If you choose not to create tokens, the ability will trigger again the next time the condition is met. Once you choose to do so, the ability will no longer trigger for the rest of the turn.

There is an inherent triggered ability associated with having rad counters. This triggered ability has no source and is controlled by the active player. The full text of this ability is “At the beginning of the precombat main phase of a player with rad counters, that player mills 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.”