From Halopedia, the Halo wiki
Shrine defenders resemble enormous dark grey metal towers with glowing portals near the top, resembling eyes. These eyes emit extremely bright orange light, capable of lighting up large expanses of land from far away. Notably, their darker metal and more brutalist construction suggest a similar origin to that of the structures found within the Sentinel wall and quarantine zone of Installation 05.
The shrine defenders are typically constructed in groups of six, pointing inwards towards select stone structures in the middle of desert expanses. When attempting to leave the predetermined "zone" in which they protect, they are capable of firing extremely powerful orange directed-energy weapons from their "eye".
The defenders were built on Installation 00 and at least one other planet, defending similarly-built areas of stone structures in the desert. At some point likely during the Battle of Installation 00, the Jiralhanae occupied the zone occupied by the shrine defenders on the Ark, though the Brutes were later forced out of the region by forces of the UNSC Aegis Fate.
Shrine defenders are found on the Halo 3 multiplayer maps Sandtrap and Sandbox. The towers first seen on Sandtrap, and named in game files as
The defenders return on the Mythic Map Sandbox, though still named the same internally. On Sandbox however, the defenders fire gauss cannon-like projectiles at players who attempt to escape the map. Prior to this lethal response, the defenders fire a form of battle rifle-like weapon at players. When this weapon hits a player, it flashes their screen yellow to alert them to their impending demise.