Historical Rules · 4 February 2011
Rule 115.2f
Official source text from the 4 February 2011 Rules release
In a Planar Magic game, rolling the planar die is a special action. A player can take this action any time he or she has priority and the stack is empty during a main phase of his or her turn. Taking this action costs a player an amount of mana equal to the number of times he or she has previously taken this action on that turn. See rule 901, “Planar Magic.”