Sandworm

The sandworm is an Easter egg on the Halo 5: Guardians multiplayer map The Rig.

Walkthrough
To activate the Easter egg you need 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 Easter egg 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.