Wurm

Wurm CR: 14

Huge monstrosity, unaligned

Armor Class: 18 (natural armour)

Hit Points: 200 16d12+96

Speed: 50 ft , burrow: 30 ft

STR

24 +7

DEX

10 +0

CON

22 +6

INT

3 -4

WIS

12 +1

CHA

4 -3

Saving Throws: Con +11, Wis 6

Senses: Blindsight 60 ft, Tremorsense 60 ft, passive Perception 11

Challenge Rating: 14

Earth Tremors. The wurm creates earth tremors as it moves overland or underground. Any creature that comes within 30 feet of the moving wurm for the first time on a turn must succeed on a DC 20 Dexterity saving throw or take 10 3d6 bludgeoning damage and fall prone. Any structure or object anchored to the ground that comes within 30 feet of the moving wurm for the first turn on a turn takes 10 3d6 force damage.
Siege Monster. The wurm deals double damage to objects and structures.
Tunneler. The wurm can burrow through solid rock at half its burrow speed and leaves a 10-foot-diameter tunnel in its wake.

Actions

Bite. Melee Weapon Attack +12 to hit, reach 10 ft, one target. Hit 24 5d6+7 piercing damage. If the target is a Medium or smaller creature, it must succeed on a DC 20 Dexterity saving throw or be swallowed by the wurm. A swallowed creature is blinded and restrained, has total cover against attacks and other effects outside the wurm, and takes 17 5d6 acid damage at the start of each of the wurm's turn. If the wurm takes 30 damage or more on a single turn from a creature inside it, the wurm must succeed on a DC 20 Constitution saving throw at the end of that turn or or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the wurm. If the wurm dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement, exiting prone.

Wurms are huge creatures that resemble limbless, wingless dragons. They burrow through the earth and eat virtually anything they come across. Normally found underground in Ravnica, a few have escaped to other Material Plane world, where they burrow straight into the earth.
A wurm burrows through through loose earth by using sonic vibrations to liquify the earth in front of it and swim through the area. The soil resolidifies and closes behind it. Moving through rock is slower and more difficult, and the wurm leaves a tunnel in its wake. 
The Great Clans of Ravnica appreciate the devastation wurms can create, and the clans sometimes lure them into civilized areas where the destruction can be vast.

Suggested Environments

Underdark, odd appearance above ground, Ravnica.