Monstrous Compendium Vol. 1: Spelljammer Creatures
Frozen Sick
D&D Beyond Basic Rules
Basic Rules (2014)
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Monstrous Compendium Vol. 1: Spelljammer CreaturesFrozen SickD&D Beyond Basic RulesBasic Rules (2014)Sign in to view your library & saved favorites.
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Is that what happens? Or do they play Mime in Invisible Box because they think they're contained? Or does trying to touch the cage count as taking the study action and if they succeed they realize it's not actually there, if they fail they're trapped?
They are no more trapped than they could walk over a bridge if you made them believe there was a bridge across a chasm. The cage does not exist thus they are not trapped, if they make any attempt to escape they succeed in doing so, just as if they attempt to cross the bridge they fall through into the chasm. It is right there in the spell description.
The core difference there is: the affected character is not moving themselves through the bridge, they are being moved by an external force (gravity). Likewise, an illusory cage would not prevent some external force from grabbing the character and moving them -- phantasmal force only affects what the target can do of their own will.
I think it depends on how they try to escape and what the bindings are. if they are in a cage and shoulder rush it they end up out of the cage. if they try to smash it with their hammer it passes through harmlessly, and they rationalize why, they might mime in an invisible box because they know force cage can't be broken, though I'd consider either of those a study action. if targeting the player I might roll for them or use passive. What the illusion is might shape the logical break free attempt is. Lets say your legs are manacled to the wall, do you just pull, do you smash it with a weapon who knows. But if you summoned a steel cage with dangerous looking spikes jutting out of it, is shoulder rush the option they would choose or would they try to break it with a weapon and keep some distance. You have now narrowed down the likely responses to something where they probably remain trapped.
That completely depends on what the enemy is. A horse would panic at being confined and would fling hooves at a dangerous looking steel cage, or bolt out of it. An intelligent humanoid wouldn't. An wizard with access to Misty Step most likely would simply teleport out of it. But generally most creature that find themselves confined will make some attempt(s) at escaping and especially during a combat situation are unlikely to just sit back and chill until they are released. Phantasmal Force is not Domination, the affected enemy can willfully do whatever they normally would do when confined. They can move as they will, they can attack as they will, they can speak as they will regardless of what the phantasm is. Since only the affected target can see the phantasm it the target has intelligent allies with them, most likely their first response will be to shout out "Help me! I'm trapped!" to which the intelligent allies will respond "No you aren't, there's nothing there, come help us kill these guys!" at which point most creatures are likely to attempt moving through the phantasm.
If you want to restrain an enemy, use Entangle, or Hold Person, or Earthen Hand, or Web, or any of the other spells that actually conjure physical restraints.
There are not specific rules for what Phantasmal Force can and cannot do, because that would completely destroy the creativity that makes the spell fun for both the DM and the players. Consider the following possible use cases: