Subrule · Chapter 1
cr:1.8.6a
If two or more players are instructed to determine the parameters of an effect, those players determine the parameters in clockwise order, starting with the controller of the effect.[1.1.6]
Example: Codex of Frailty has the text “Each hero puts an attack action card from their graveyard face-down into their arsenal,” which requires all players to determine what card will be selected in their graveyard to be put into their arsenal. This starts with the player who controls Codex of Frailty as it resolves on the stack.