Truth Gag

You create a magical bind that prevents the spell’s target from revealing any secret information they have gained within a ‘safe space’ designated in the casting of the spell. Until dispelled, a creature that is bound by the spell's affect can not physically speak any knowledge gained in the safe space.

A creature can attempt to save against the spell taking affect by making a Charisma saving throw (which the target can chose to fail). On a failed save a creature can't utter any knowledge gained in the safe space to anyone who is not also subject to the truth gag spell for the same safe space. The caster will know whether each creature succeeds or fails on its saving throw.

An affected creature is aware of the spell and can thus avoid answering questions which it can not answer. Such a creature can be evasive in its answers as long as it remains within the boundaries of the spell.