The active player has Lethal vapors on the field. During his main phase he casts Teferi's Protection , holds priority and activates the ability of Lethal Vapors an arbitrary large number of times. Opponent realizes that this will cause him to take lots of turns while he is unable to do anything and eventualy lose the game for drawing from an emty library. Because of that in response to all of that he activates the ability of lethal vapors a number of times equal to the active player. In response to that the active player activates the ability lots of times again. The players start throwing bigger and bigger numbers at themselves and finaly call a judge to resolve the issue, because this state isn't actualy going anywhere.

What should be our answer, asuming Regular REL?

Hi Andrzej,

Thanks for your question. This falls under the loop rules (specifically, 719.3):

719.3. Sometimes a loop can be fragmented, meaning that each player involved in the loop performs an independent action that results in the same game state being reached multiple times. If that happens, the active player (or, if the active player is not involved in the loop, the first player in turn order who is involved) must then make a different game choice so the loop does not continue.

In short: the active player will have to choose a number of times to activate, then the nonactive player (knowing that number) can activate it more times, and then we're done (the active player is not allowed to activate it anymore). In this case, the active player (the one casting the Protection) will have to stop before their opponent does.

Thanks,
Nathan Long
Magic Rules NetRep