Can a hidden unit (like Teemo) be played from a battlefield when Mageseeker Warden is preventing units from being played to battlefields?

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Accepted Answer

No. A card hidden at a battlefield must be played to that battlefield [811.1.d.1], and Mageseeker Warden’s effect prevents opponents from playing units to battlefields, so you cannot play the hidden unit from Hidden to that battlefield while the Warden’s restriction is active. You could instead play the card normally from hand (not from Hidden) per [811.3], but that play would be subject to the Warden and thus must go to your base.

You may not play a unit from Hidden at a battlefield to that battlefield if Mageseeker Warden is active and causing opponents to only be able to play units to their base. The Hidden rule requires the played permanent to be placed at the battlefield where it was hidden [811.1.d.1], and the Warden prohibits playing units to battlefields for opponents, so there is no legal way to place it at that battlefield from Hidden. The player may instead choose to play the card from hand (not from Hidden) per [811.3], in which case they must play it to base due to the Warden.

Identify that the unit card is facedown at a battlefield as a Hidden card (it was previously Hidden there). Attempting to play it from Hidden requires placing that permanent at the battlefield where it was hidden per [811.1.d.1]. Check whether that battlefield is a legal play destination for the player. By default valid locations include Base or a Battlefield you control [355.2.a]. If Mageseeker Warden is at a battlefield and its text is preventing opponents from playing units to battlefields (allowing only base), the battlefield where the card is hidden is not a legal place for the opponent to play a unit to while that restriction applies. Because playing from Hidden requires placing the permanent at that battlefield and that battlefield is not a legal destination, the hidden play cannot be put on the chain / cannot be played from Hidden. The player may instead play the card from their hand for its printed cost (not using Hidden) per [811.3]; that play will follow normal location rules and so must be to base while the Warden’s restriction is active.

If the Hidden card’s play effect explicitly makes playing to that battlefield impossible (see [811.1.d.2]), the play-from-Hidden may be allowed to target or move to other locations as described by that effect. In such a case the Hidden card’s play could be legal even with Mageseeker Warden present, but only to the extent the Hidden card’s own wording allows a different destination. If Mageseeker Warden leaves the battlefield (or its effect ceases) before the Hidden card is played, the battlefield may become a legal destination and the Hidden card can be played there. (Control and timing interactions that preserve battlefield control while items are on the chain are governed elsewhere; but the core result is that the destination must be legal at the time you finalize the play.) If the player chooses to not play the card from Hidden but instead plays it normally from hand (using [811.3]), that play is legal but will be restricted by the Warden to go to base.

The Hidden keyword places a strong restriction on how a facedown Hidden permanent is played: it must be played to the battlefield where it is hidden [811.1.d.1]. Playing a unit requires selecting a valid location for that unit (valid locations are by default your Base or a Battlefield you control) [355.2.a]. Mageseeker Warden’s continuous effect that "opponents can only play units to their base" makes battlefields illegal destinations for the opponent’s unit plays while it is active. Those two pieces of text conflict for a Hidden unit that was hidden at a battlefield the opponent controls: Hidden demands that the card be played to that battlefield, but the Warden forbids playing units to that battlefield. Because you cannot meet the Hidden requirement and the Warden’s restriction at once, you cannot play the unit from Hidden in that situation. The only straightforward workaround is to not play it from Hidden: per [811.3], instead of being Hidden you may play the card for its printed cost as a normal play, which carries no special battlefield-placement requirement from Hidden. That normal play would then be constrained by Mageseeker Warden and must be played to base while the Warden’s effect applies. Note there is an exception if the Hidden card’s own play effect explicitly makes a battlefield-targeting restriction impossible and thus allows a different destination (see [811.1.d.2]); in such a case the specifics of that card would govern whether the play-from-Hidden can legally be performed despite the Warden.