Covenant rope
From Halopedia, the Halo wiki
The Energy Tether is a device used by Covenant soldiers to rapidly descend from high places. They work similar to rappelling rope, but are made of pure energy. It is unclear how the ropes are kept together or how they are powered, but they appear to work just as well as normal rope without causing rope-burn if descended too rapidly. The most common usage of these ropes are by Elites and Jackals.
Covenant soldiers have been known to use energy rope-like tethers to climb, and descend steep surfaces.