Source: Eberron - Rising from the Last War

Wondrous item, common (requires attunement by a creature with the Mark of Scribing)

You can use this pen to write on any surface. You decide whether the writing is visible or invisible, but the writing is always visible to a person with the Mark of Scribing.

Any creature with the Mark of Scribing can use an action to touch the invisible writing, making it visible to all.

If you use the pen to write on a creature that isn't a construct, the writing fades after 7 days.