This is a magic item I introduced in a homebrew game several years ago and it resulted in a ton of fun for the players. The party discovered a strange, obviously magical tablet mixed in with some mundane items in a gnoll's lair. One night, the player who took the tablet noticed a message that had appeared on it. She wiped it clean, responded with a piece of chalk, and another message appeared. We used a small white board that we sent back and forth to simulate the tablet and it ended up being really immersive and a fun mystery for the party. Eventually, it was revealed that a hag controlled a second, linked tablet. The hag ended up being a tenuous and nefarious ally when the party needed help with something - and always for a price.
Tablet of Messaging
Wondrous item, uncommon
These magical tablets are always created in pairs, though lost to war and time, few are still together. The Tablet of Messaging is linked to a second tablet. As long as the two tablets are on the same plane of existence, a creature can write a message on the tablet which will appear on the linked tablet. Creatures can use their hand to swipe away a previous message and write a new one. Each tablet includes a piece of chalk that magically replenishes each night at midnight, seemingly magnetically affixed to the tablet.