Cards And Zones
Where Can You Play a Unit or Your Chosen Champion?
Choose your Base or a battlefield you control, unless a card gives another permission.
RiftPal explanation · Answer first
The short answer
Normally, play a unit to your BaseYour Base is your personal location where your units can be played and return to during the game.View in Glossary or a BattlefieldA Battlefield is a shared location that players compete to control during the game.View in Glossary you control. Choose the location before paying. The unit enters exhausted unless an effect says otherwise.
Your Chosen ChampionThe champion unit selected during deckbuilding to start in your Champion Zone. Its champion tag must match your legend. During play, champion units with that same full name also count as your Chosen Champion.View in Glossary in the Champion ZoneThe separate zone where your Chosen Champion starts the game. You can play it from there using the normal timing, costs and restrictions. It is not a location on the board.View in Glossary follows those same rules: you still pay its cost and choose a legal location.
RiftPal explanation
How it works
Card reference
Darius, Trifarian
The Champion ZoneThe separate zone where your Chosen Champion starts the game. You can play it from there using the normal timing, costs and restrictions. It is not a location on the board.View in Glossary is an additional place you can play that copy from. It does not make the unit free or let it enter anywhere. Other copies in your Main DeckThe deck of units, gear and spells, separate from your Rune Deck. Your deckbuilding list has at least 40 cards including your Chosen Champion; that chosen copy starts in the Champion Zone instead of the shuffled pile.View in Glossary are played normally from your hand when drawn.
Champion Unit
Darius
Trifarian
When you play your second card in a turn, give me +2 [Might] this turn and ready me.
AmbushPermission to play a unit to a battlefield where you control units, with Reaction timing when playing it there. Other play requirements still apply, and Ambush alone does not make the unit enter ready.View in Glossary gives a unit another option: a battlefield where you control units, even if you do not control that battlefield. When played there, it also has ReactionA timing keyword that includes Action's permissions and also permits a card or activated ability while a Chain is waiting. You still need the proper opportunity, normally Priority; it does not allow an interruption halfway through resolution.View in Glossary timing. You still need a legal opportunity to respond, and the permission must remain valid through the play's legality check. Ambush alone does not make the unit enter ready.
In games with more than two players, you cannot choose to play a unit into a CombatCombat is the part of play where opposing players' units fight for the same Battlefield.View in Glossary between other players, including one that is staged but has not started. If an effect requires that placement, the unit enters your BaseYour Base is your personal location where your units can be played and return to during the game.View in Glossary instead. No choice may create a Combat involving more than two players.
In practice
Example
You control a battlefield with one of your units there. You can play a unit from your hand directly to that battlefield, paying its cost and placing it exhausted.
You could choose your Base instead. You cannot choose an empty battlefield you do not control without a separate permission.