Scribe's Pen

From Eberron: Rising From the Last War, page 278 and; Wayfinder's Guide to Eberron, page 115.

Description
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.

Wayfinder's Guide to Eberron
Consult your DM before using this version. It is usually considered unofficial and invalid due to updated material.

If you possess the Mark of Scribing, you can use this quill to write on any surface. This can be visible — traced in glowing mystical lines — or invisible to any creature without the Mark of Scribing. Invisible writing will be revealed by Detect Magic, See Invisibility, or True Seeing. Any creature with the Mark of Scribing can also reveal your writing or make it invisible as an action. If you mark a living creature, the mark will fade within a week.