Movement
When Do You Gain or Lose Control of a Battlefield?
Control changes after the game settles a contest, not merely because another unit arrives.
RiftPal explanation · Answer first
The short answer
You establish control after a ShowdownA Showdown is a structured opportunity for players to act when a Battlefield is contested.View in Glossary or CombatCombat is the part of play where opposing players' units fight for the same Battlefield.View in Glossary leaves only your units at the BattlefieldA Battlefield is a shared location that players compete to control during the game.View in Glossary . Moving or playing a unit at a battlefield you do not control normally makes it ContestedA temporary battlefield status applied when a unit becomes present there, its controller does not control that battlefield, and it is not already contested. It prepares a Showdown or Combat rather than immediately transferring control.View in Glossary ; it does not immediately give you control.
RiftPal explanation
How it works
A BattlefieldA Battlefield is a shared location that players compete to control during the game.View in Glossary is occupied whenever any unit is there. It can still be controlled by another player while ContestedA temporary battlefield status applied when a unit becomes present there, its controller does not control that battlefield, and it is not already contested. It prepares a Showdown or Combat rather than immediately transferring control.View in Glossary . The existing ControllerThe player who currently controls a game object and makes the choices assigned to that controller. Control is established differently for battlefields and other objects, and need not match ownership.View in Glossary keeps control until the ShowdownA Showdown is a structured opportunity for players to act when a Battlefield is contested.View in Glossary or CombatCombat is the part of play where opposing players' units fight for the same Battlefield.View in Glossary procedure checks control. At that check, the player whose units remain establishes control if no further Showdown or Combat is staged there. If no units remain, the battlefield becomes uncontrolled.
Outside a Showdown or Combat, a controller who no longer has units there loses control at the next cleanup in an Open State, once the ChainAn off-board zone that holds cards and abilities during the process of playing them. Cards and abilities on it are public information.View in Glossary is empty. Having a unit at an uncontrolled battlefield can start a Showdown, and closing that Showdown with only your units there establishes control.
Card reference
Forgotten Library
The BattlefieldA Battlefield is a shared location that players compete to control during the game.View in Glossary 's ControllerThe player who currently controls a game object and makes the choices assigned to that controller. Control is established differently for battlefields and other objects, and need not match ownership.View in Glossary normally controls its abilities and makes their choices. If the battlefield is uncontrolled, the Turn PlayerThe player taking the current turn.View in Glossary handles an ability unless it names another player to do so. But "you" still refers to the battlefield's controller: with no controller, instructions addressed to "you" are ignored. In a team game, control is not shared with your teammate.
Battlefield
Forgotten Library
While you control this battlefield, when you play a spell, if you spent [Energy 4] or more, [Predict]. (Look at the top card of your Main Deck. You may recycle it.)
Establishing control can also be a ConquerConquer means taking control of a Battlefield you did not already control and Scoring it for the turn.View in Glossary if you have not already scored that battlefield this turn.
In practice
Example
You control a battlefield with one unit. An opponent moves a unit there, making it Contested, but you remain the controller until the Showdown or Combat checks control. If only the opponent's units remain afterward, that opponent establishes control and may Conquer if they have not scored that battlefield this turn.