Sentry Chalk
Wondrous Item, Common (No Attunement)
The chalk has 3 charges and regains all expended charges at dawn. Spend 1 minute and 1 charge drawing up to six arrows at a passage junction, each pointing along a visible branch. Name up to six creatures present as exempt.
For 24 hours, whenever a Small or larger creature other than an exempt creature moves toward the junction along a marked branch within 100 feet of the drawing, that branch's arrow turns red for 1 minute. Measure distance along the passage. Invisible creatures trigger the arrow. Further qualifying movement restarts its minute. Detection works through intervening barriers along that passage. A red arrow identifies only the branch where movement was detected. Reading it requires seeing the drawing.
The marks work underwater. Moving their surface away from the junction or scrubbing the drawing for 1 minute ends the effect. Its arrows then become ordinary white chalk.