Another creature from my current fantasy roleplaying campaign, which the PCs fought.

Fleshwyrms are the non-sapient rape-children of two of the outcast gods; Choren, the god of Wrath, is the father, and Kothrin, the god of Torment, is the mother. Kothrin repeatedly attacked Choren's Iron Citadel, with thousands upon thousands of demons and hundreds of thousands of lost souls being killed in these battles. Choren was always a better fighter and commander, being one of the war-gods, and would always win. Despite their violent interactions there wasn't any deep hatred or resentment. Choren loved the opportunity to fight a powerful opponent, beating another god, and greatly enjoyed beating Kothrin down and brutally raping her. Kothrin enjoyed the carnage and struggle, and absolutely loved being beaten and raped so viciously, something that very few beings were capable of. They have a strange relationship...

In any case, she kept staging her little invasions, he kept beating her down, and in the aftermath of the battle she would give birth to a child. Surrounded by the massive energies of death and born onto battlefields where millions die each year, they were warped into animalistic, vicious predators. They made these battlefield their home, and could burrow through the heaps of mangled bodies like an eel through water. Some of these battlefields, such as the Field of Blood before the Iron Citadel, could give huge amounts of area for these fleshwyrms to live in. Hemmed in by steep cliffs, this field measured only a few square miles, but the constant warfare on Wrath has brought so much death that the bodies of the dead now form the only ground, reaching over a hundred feet in depth, and dozens of other battlefields reach similarly impressive depths.

Being the offspring of a god, fleshwyrms are terrifying creatures. They can reach lengths approaching 200 feet, and can weigh over 100 tons. Not only are they incredibly resistant to injury, they also heal wounds with incredible speed. They move with frightening speed on the surface, able to easily out-pace a horse, and are even faster while burrowing through the corpse-piles of their homes. They can feel vibrations in the ground so well that they can pinpoint the location of anyone moving within hundreds of yards, as well as determining their size, weight, speed, and more. There is no warning when they attack, with the first sign of their presence being their massive jaws bursting from the surface to bite down on their prey, or their many claws reaching up and dragging their target under the seemingly-solid surface.

Despite being of only animal intelligence, they are every bit as sadistic and violent as their parents, and take great pleasure in terrifying and torturing anyone they can, particularly lone prey. Someone alone in their territory might be suddenly grabbed, surrounded by the fleshwyrm's coils, and slowly picked to pieces with slow, precise nips. Even when they have to snatch their prey quickly and depart, they enjoy dragging their prey under to suffocate deep under the rotting corpses, or swallowed whole and helpless.

...though technically what the PCs fought wasn't a fleshwyrm. It was a succubus who had taken the form of a fleshwyrm. Basically, all the bad stuff of a normal fleshwyrm, but add in the ability to shapeshift, fly, and mentally assault anyone nearby. She was unpleasant.