From Halopedia, the Halo wiki
A stealth pylon is a piece of Covenant technology that is capable of hiding large areas from sensors.
The pylon is a large tower-like object with a revolving head that releases a pink glow every few seconds. They are supported by three tripod-like legs, which each contain the pylon's energy supply systems. Underside of the pylon is a gravity lift which can lift personnel up to the main level which mounts three plasma turrets for defense.
Several stealth pylons were used in the Viery Territory during the opening stages of the Fall of Reach to shield large Covenant deployments. One such pylon was discovered by Jun-A266 and SPARTAN-B312 during a reconnaissance mission to Szurdok Ridge. Jun planted a remote detonator charge on the pylon. The next day, when a UNSC strike force moved in to attack the Covenant landing zone, Kat-B320 triggered the charge, destroying the pylon.
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