Sandworm

The sandworm is a massive creature living underneath the surface of the human outer colony of Meridian. The sandworm seemingly survived the Covenant's glassing of Meridian in 2551. Despite Free Frontier Education's The History of Meridian claiming the Battle of Meridian killed all life on the moon the animal was known to be simulated aboard the for War Games.

Easter egg walkthrough
The sandworm is only visible as an easter egg on the Halo 5: Guardians multiplayer map The Rig. To activate the Easter egg a player needs to ground pound a platform above the hole in the middle of the map, which will cause two canisters to come out of silos. When the canisters are shot a sandworm will come out of the sand and swallow a distant rig.

Trivia
The sandworm is a reference to the science fiction series Dune, specifically the sandworms which roam the desert planet Arrakis and are known to devour mobile mining rigs whole.

List of appearances

 * Halo 5: Guardians