Choose a source you control and flip a coin. If you win the flip, the next time that source would deal damage this turn, it deals double that damage instead. If you lose the flip, the next time it would deal damage this turn, prevent that damage.
The chosen source may be a permanent, a spell on the stack, or anything referred to by something on the stack (such as a Prodigal Pyromancer that’s left the battlefield while its ability is still on the stack)