Historical Rules · 25 August 2017
Rule 718.5a
Official source text from the 25 August 2017 Rules release
At the end of a subgame of a Planechase game, each face-up plane card or phenomenon card is turned face down and put on the bottom of its owner’s planar deck. Then each player moves his or her planar deck from the subgame command zone to the main-game command zone and shuffles it.