I could see this as a D&D sub-boss.

You are all still walking down this canal when there is an exit to the passage.

In front of you is a cloudy silhouette of what you can discern is a grouping of a few very large buildings with a noticeable closer bulbous looking building with two humongous round obsidian spheres on the top.

Unfortunately there seems to be a field of jagged stones placed randomly along ridges surrounding said building.

~Maybe half a day later of walking in and out of the ridges one of the players makes a perception roll to get their bearings within the jagged rocked ridges.

Although none of the rocks you passed were the same looking you notice that some of those rocks behind you have been moved by someone or something.

~A few minutes pass in game, make a player or all of them roll another perception check.

You all now notice with horror that all the rocks, both near and far, have all moved. What is worse now is that the two gargantuan round obsidian spheres are looking down, fixed on you all.

While you all stop dead in your tracks, it is all too late you all notice the sound of slow rumbling as the closest ridge is now actively moving, you all peer up as below the black eyes of this massive snake a forked tongue juts in and out of it's mouth, smelling the meat below it.