Terravorax
"Godzilla Hanna-Barbera EarthEater" © deviantArt user "UltimateDitto", accessed at his gallery here
[Sponsored by @glarnboudin, his third sponsorship inspired by the Hanna Barbera Godzilla series. He suggested I focus on its tunnel making as a Watsonian explanation for why there's so many big open spaces underground, similar to the poor maligned delver of D&D 3x and PF1e. The Earth Eater is a monster with a very Weaksauce Weakness (to quote TVTropes), so I intentionally made its hit points and save DCs a bit on the high side for a monster of its CR.
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Terravorax CR 19 N Magical Beast This immense creature resembles a tegu larger than a building, with a blocky skull and underslung jaw. A row of stony plates runs down its back, and it has a slotted club, almost like a gear, growing from the tip of its tail. Two small antennae grow from its brow, above its heavily-lidded eyes.
Terravoraxes, also known as eartheaters, are enormous reptilian monsters that live and feed underground. They derive nutrition from literally eating soil and stone and are frequently on the move in search of unusual minerals, fossils and other tasty treats. This can cause problems—soil pollution and mining can drive them towards the surface, where they then find “minerals” such as brick and masonry to feast on. A terravorax can level a city in a feeding binge, after which it burrows back into the depths to rest.
A terravorax is more interested in feeding than fighting, but if they are attacked by people defending their homes will fight back savagely. Their jaws and tail are devastating enough, but their greatest weapon is a blast of sonic energy fired from the two antennae that grow on their head. Not only does this blow apart creatures and objects caught directly in the line of fire, it causes tremendous earthquakes. Perhaps the greatest blessing when fighting a terravorax is their relatively common weakness—plain water melts through their earthen bodies the way acid corrodes flesh.
Terravoraxes leave large, stable tunnels in their wake as they dig through rock, and many of the caverns of the Darklands were either made or expanded through the actions of a terravorax. Unsurprisingly, dwarves, drow and other humanoids living in subterranean realms view reports of a terravorax as ill omens, and may send adventurers out to slay the monster or divert its attention away from an underground settlement. A handful of times in their very long lives, a terravorax is compelled to mate. Females lay their eggs near the volcanic heat of a magma chamber, but otherwise perform no parental care.