What happens when Here to Help is played from hidden (via Rockfall Path) if the player doesn't control a battlefield where units can be played?

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Accepted Answer

You may play Here to Help from Hidden on Rockfall Path. Because the spell’s unit play is optional and units in hand are not targets, you can simply choose not to play a unit to that battlefield; the spell resolves and does nothing else. The Hidden-play restriction that prevents playing a spell with no valid targets does not stop this, because Here to Help does not require a legal target to be played from Hidden.

You can play Here to Help from Hidden to the battlefield it was hidden at (Rockfall Path). During finalization you may choose to play a unit from hand to that battlefield, but since Rockfall Path has “Units can't be played here” you have no legal unit-play option there and may decline the optional play. The spell resolves with no unit being played.

You attempt to play Here to Help from Hidden; it must be played to the battlefield where it was hidden (Rockfall Path) [811]. When finalizing the spell, you make the spell's choices (including whether to use its 'You may play a unit from hand...' option) [355.1]. The option would require the unit to be played to the same battlefield it was hidden at [811.1.d.3]. Because Rockfall Path has 'Units can't be played here,' there is no legal unit you can play to that battlefield, so you may decline the optional play. The spell then resolves; no unit is played and the spell has no further effect.

If another effect at finalization makes that battlefield temporarily able to accept units (or makes some other battlefield a legal destination for the unit play), you must follow the Hidden restriction and play the unit to the battlefield where the card was hidden [811.1.d.1] [811.1.d.3]. If the spell's text targeted a specific unit at that battlefield as part of being played from Hidden (not the case for Here to Help), and that target didn’t exist or was illegal at finalization, then the spell could be unplayable from Hidden per the 'no valid targets' restriction [811]. If you had not hidden the card and instead played Here to Help normally, you could play a unit to any battlefield you control where units are allowed — the Hidden-specific battlefield restriction would not apply [811.3].

When a card with Hidden is played from face down, it must be played to the battlefield it was hidden at [811]. The Hidden rules also say that if a hidden spell or a play effect of a hidden permanent causes you to play a unit, you must choose to play that unit at that battlefield [811]. However, the Hidden rule that prevents playing a spell from Hidden only applies when the spell has no valid targets under the battlefield-restricted targeting rules; that is, Hidden can bar a spell that needs a target at that battlefield but has none [811].

Here to Help’s text reads 'You may play a unit from hand to a battlefield you control, reducing its cost...' The 'you may' makes the unit play optional, and crucially playing a unit from hand does not target a specific card in hand because hands are non-public zones (so the 'no valid targets' Hidden prohibition doesn’t apply here) [355]. When you finalize the spell, you choose whether to exercise that optional play [355.1]. If the spell was hidden on Rockfall Path and that battlefield disallows unit plays, there is simply no legal way to carry out the optional play to that battlefield [811.1.d.3], so you may decline to play a unit. The spell itself can still be played from Hidden and will resolve; it will just do nothing in regard to playing a unit.

Therefore, the most defensible ruling under the provided rules is: you may play Here to Help from Hidden on Rockfall Path, but you cannot play a unit to that battlefield (and may decline the optional play), so the spell resolves without playing a unit.