Tunneler. The worm can burrow through soil at half its burrow speed and leaves a 5-foot-diameter tunnel in its wake.
Monstrous Regeneration: When a creature dies while consumed inside of the Aspect of the Devourer their body is dissolved within the acid of the worm. When a creature dies in this way the worm gains HP equal to a hit dice associated with the size of the creature (1d4 small. 1d6 medium, 1d10 large, 1d20 huge). Additionally the HP gained in this way is also added to the Worm's maximum HP.
Looming Proportions: As the Aspect of the Devourer continues to feed so too does its form grow. For every 25 HP that the worm's maximum HP grows it gains 1 Strength and 1 Constitution. For every 50 HP that is gained in this way the Worm's form swells in size and grows one size larger. Associated with that the damage dice for the bite attack increases and the acid damage increases as well as other things. (see numbers in parentheses.)
Dark Blessings: As the power of the Aspect of the Devourer continues to grow it gains 1 legendary resistance each time it grows in size.
Unending Hunger: With the Devourer's blessing also comes the insatiable hunger to feed. If the servant of the Devourer were not to consume a creature in this way for a week the proportions of the worm would begin to diminish. Every week nothing is consumed the worm grows one size smaller and suffers a point of exhaustion. This can only reduce the size to that of its base stats. As the size diminishes place the devourer at the appropriate maximum HP reference to the size. Additionally when the size is reduced the player suffers a point of exhaustion on the first day of that week. This point of exhaustion continues to increase one point for each week that passes.
Multiattack. The worm makes two attacks
Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 13 (2d8 + 5) piercing damage. If the target is smaller than the Aspect of the Devourer, it must succeed on a DC 15 (16,17) Dexterity saving throw or be swallowed by the worm. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the worm, and it takes 4D6 (5D6, 6D6) acid damage at the start of each of the worm's turns.
If the worm takes 20 (25, 30) damage or more on a single turn, the worm must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the worm. If the worm dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 10 (15,20) feet of movement, exiting prone.