The Hippodrome

All creatures get -5/-0.
Whenever chaos ensues, you may destroy target creature if its power is 0 or less.

Planechase Anthology Planes

All creatures get -5/-0.
Whenever you roll {Chaos}, you may destroy target creature if its power is 0 or less.

Planechase Planes

All creatures get -5/-0.
Whenever you roll chaos, you may destroy target creature if its power is 0 or less.

A creature that would assign 0 or less combat damage (due to having power 0 or less) doesn't assign combat damage at all.

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.

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."

Creatures may have negative power. For example, a 3/4 creature that gets -5/-0 becomes -2/4. The total of its power and toughness is 2. It would need +3/+0 to raise its power to 1.

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.

The controller of a face-up plane card is the player designated as the "planar controller." Normally, the planar controller is whoever the active player is. However, if the current planar controller would leave the game, instead the next player in turn order that wouldn't leave the game becomes the planar controller, then the old planar controller leaves the game. The new planar controller retains that designation until they leave the game or a different player becomes the active player, whichever comes first.

You may target any creature with The Hippodrome's chaos ability. Whether its power is 0 or less isn't checked until the ability resolves.