Undercity Reaches

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.

The chaos ability's effect continues to apply even after Undercity Reaches stops being the face-up plane card.

If a creature deals combat damage to a player, the controller of Undercity Reaches controls the ability that triggers, but the controller of the creature decides whether to draw a card.

If multiple effects modify your hand size, apply them in timestamp order. For example, if you put Null Profusion (an enchantment that says your maximum hand size is two) onto the battlefield and then roll {CHAOS} while Undercity Reaches is the face-up plane card, you'll have no maximum hand size. However, if those events had happened in the opposite order, your maximum hand size would be two for as long as Null Profusion is on the battlefield.