If two different replacement effects want to replace the same event, who chooses which applies first?

When two or more replacement effects would modify the same event, the game must determine which one applies first. According to CR 616.1, if two or more replacement effects would replace the same event, the affected player or the controller of the affected permanent chooses which replacement effect to apply first.

After the first replacement effect modifies the event, the game checks again to see if any remaining replacement effects still apply to the modified event (CR 616.1). If they do, the same player continues choosing the order. This process repeats until no more replacement effects are applicable.

There is an important exception: some replacement effects have a built-in priority. Specifically, self-replacement effects — those that replace an event that would happen to the object with the effect itself — are applied before other replacement effects that would modify the same event (CR 616.1d). Similarly, prevention effects and redirection effects follow specific ordering rules under CR 616.1a–616.1d.

Concrete example: A player controls a Leyline of Sanctity (which makes them an illegal target) and has Platinum Emperion in play (their life total can't change). If a spell would cause that player to lose life, and both a life-loss prevention effect and another replacement effect apply, the affected player chooses which replacement effect to apply first, potentially making the second one irrelevant if the event has been sufficiently modified.

In short: the affected player (or permanent's controller) is always in the driver's seat when choosing the order of competing replacement effects, giving them meaningful strategic control over outcomes.