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 grappelling 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. It is most likely held together by the same force that holds an Energy Sword together. Alternatively, the Energy Tether may be similar to a small but extremely powerful Gravity Lift. The most common usage of these ropes is by Elites and Jackals.
Covenant soldiers have been known to use energy rope-like tethers to climb and descend steep surfaces. But Brutes don't seem to use them.
