Rock-marking pen

In almost complete darkness of The Depths, leaving marks for other travelers tempted to traverse those layers can be pretty tricky. Anything clearly visible that's easy to wipe off, destroy, or scratch off will probably be destroyed by beasts living down here, once one of them manages to find it. It all became way easier the day the rock-marking pens have been created.

A simple design...

Rock-marking pens are black smooth tubes, narrow on one side topped with a nib. They vary in length, depending on the strength of the solution (the ink) contained within them. The longer models let the user create deeper and brighter glowing markings. Both the barrel and the nib are resistant to the properties of pen's ink.

...Storing a powerful chemical compound

The ink used in rock-marking pens is a powerful acid capable of melting inorganic substances if exposed to oxygen. Once it touches an inorganic substance, it starts to "burrow" into it, melting it rapidly. While "burrowing", the amount of oxygen it can use drops and once it drops to a certain level, the acid neutralizes itself. The neutralization of the acid activates other ingredients of the ink which results in its luminescence. Deep glowing holes created by rock-marking pens are way harder to destroy or wipe off than simple paints or carvings.

Reactivating ink

There's a more expensive version of this invention that uses a better formula for its ink. The enhanced ink can reactivate its acidic properties should the amount of oxygen it can access suddenly rise (for example if something tries to cut into the rock to destroy the markings).

Length:
Between 10cm and 1m. There has been an order for a 2-meter one once.
Rarity:
Fairly common in The Depths, their popularity drops on the higher layers of The Pit.
Creator:
Apparently, the design of the pens and the formula for the ink have been recovered from one of the pages of the Book of Blueprints. The creator of the pens was someone from the civilisation that lived in The Pit in the past.

Dangers

There have been rumors of more intelligent beasts getting a hold of one of the rock-marking pens. According to said rumors, they managed to create markings that led people towards the danger. Either away from settlements or towards their nests. To counteract it, many tribes try to cipher their markings in a way only their members and some of their allies will understand.