Whisper seal

You inscribe a magical seal upon a scroll, book, or any written surface. The seal cloaks the text in illusion, rendering it unreadable and incomprehensible to any creature attempting to read it by normal means. The text appears as shifting runes, meaningless scribbles, or blank parchment — at your choice when casting the spell.

Only a creature who whispers the correct passphrase within 1 foot of the sealed document can pierce the illusion. Upon doing so, the illusion fades temporarily, revealing the true text for that creature for 1 minute. Once the time ends, the illusion returns unless dispelled.

If the creature does not whisper the exact phrase or speaks at a normal volume, the text remains hidden. Shouting or speaking loudly causes the illusion to flicker briefly with threatening symbols or dark shadows (purely cosmetic).

The caster determines the passphrase at the time of casting. It can be any sequence of up to 10 words, in any language the caster knows. You may choose whether or not the reader feels a subtle magical pull encouraging silence around the sealed item.

The illusion is detectable by detect magic (illusion school), and can be removed with dispel magic or true seeing. The spell ends permanently if the written material is destroyed.

 At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the duration includes an automatic alarm effect (as per the Alarm spell) that alerts you if someone reveals the text without using the correct passphrase.