Rope Arrows
From The DarkMod Wiki
Rope arrows must be shot into a wood surface to work. When shot at a non-wooden surface, the arrow will break. Otherwise, a rope will be deployed, the length being decided by how much space is under the arrow when it impacts.
Rope arrows can cause damage if fired at an opponent.
As of 1.07 AI will find a hanging rope suspicious and come to investigate.
In-game description
Rope arrow: On impact, these drill into any wooden surface and release a rope. Originally used in naval warfare, thieves have developed a more compact version for getting into hard to reach places.