Immersturm

If an ability of a plane refers to "you," it's referring to whoever the plane's controller is at the time, not to the player that started the game with that plane card in their deck. Many abilities of plane cards affect all players, while many others affect only the planar controller, so read each ability carefully.

Any "as [this permanent] enters" choices for the card returned by the chaos ability are made by that card's owner, not its old controller or the controller of the chaos ability.

A face-up plane card that's turned face down becomes a new object with no relation to its previous existence. In particular, it loses all counters it may have had.

When the chaos ability resolves, the targeted creature is exiled, then immediately returned to the battlefield as a new permanent with no relation to its previous existence.

If a creature controlled by a player other than the planar controller enters, the planar controller (not the creature's controller) controls Immersturm's triggered ability. That means that if multiple creatures enter at the same time, that player controls all of Immersturm's triggered abilities and may put them on the stack in whichever order they like. The last one put on the stack is the first one to resolve.

A plane card is treated as if its text box included "When you roll {PW}, put this card on the bottom of its owner's planar deck face down, then move the top card of your planar deck off that planar deck and turn it face up." This is called the "planeswalking ability."