Century Worm

Carapace. The worm has resistance to all damage from weapon attacks not made from within it.

Tunneler. The worm can burrow through solid rock at half its burrow speed, and leaves a 10 ft. diameter hole in its wake.

Earth Quaker. When the worm burrows 30 ft. or fewer below the surface, 10 ft. wide, 10 ft. deep fissures appear on the surface above where it passes. Creatures within the space of a fissure when it opens must succeed on a DC 20 Dexterity saving throw or fall prone in the fissure. On a success, a creature may move to an unoccupied space within 5 ft. of the fissure to avoid falling in.

Worm Keening. At the beginning of each of the worm's turns, each creature within 30 ft. of the worm takes 18 (4d8) thunder damage, and must make a DC 18 Constitution saving throw. On a failure, a creature is deafened until the start of the worm's next turn. A structure within the area instead takes the maximum amount of damage possible.

Shock Sensitivity. If the worm, or the ground within 30 ft. of the worm is subjected to lightning damage, the worm cannot use its Terror From The Deep or Swallow Earth actions on its next turn, and must use all of its movement to surface if it is below the earth.

Implacable. If the worm fails a saving throw, it can choose to succeed instead. If it does, it loses 25 hit points.

Multiattack. The worm makes two Spit Bile attacks.

Bite. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 31 (4d10+9) piercing damage. If the target is Large or smaller, it must succeed on a DC 19 Dexterity saving throw or be swallowed by the worm. A swallowed creature is blinded and restrained, and has total cover against all attacks and other effects outside the worm.

If the worm takes 30 damage or more on a single turn from a creature inside it, 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 ft. of the worm. If the worm dies, a swallowed creature is no longer restrained by it, and can escape from the corpse by using 20 ft. of movement, exiting prone.

Spit Bile. Ranged Weapon Attack: +6 to hit, range 30 ft., one target. Hit: 27 (6d8) acid damage. If the target is wearing nonmagical armor, it takes a permanent and cumulative -1 penalty to the AC it offers. Armor reduced to an AC of 10 is destroyed.

Swallow Earth. While burrowing no more than 20 ft. beneath the surface, the worm begins to swallow the earth above it. A 5 ft. radius pit appears on the surface above the worm - each creature in the area must succeed on a DC 20 Dexterity saving throw or fall in and have the worm immediately make a Bite attack against it, or move to the nearest adjacent space on a success.

Terror From The Deep (Recharge 5-6). The worm moves up to its Burrow speed, them bursts from underground, thrashing in a flurry of debris. Each creature in a 15 ft. radius must make a DC 20 Dexterity saving throw or take 35 (10d6) bludgeoning damage and be knocked Prone, or half as much damage and no additional effects on a success. As a bonus action, the worm may make a Bite attack against a creature that failed its saving throw.

The worm can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time, and only at the end of another creature's turn. The worm regains spent legendary actions at the start of its turn.

Writhing Dive. The worm moves up to half its burrow speed. Each creature that makes an opportunity attack provoked by this movement must succeed on a DC 20 Dexterity saving throw or fall into the tunnel left in its wake.

Bile Surge (Costs 2 Actions). Each creature swallowed by the worm must succeed on a DC 20 Constitution saving throw or take 22 (5d8) acid damage. On the worm's following turn, attacks it makes with its Spit Bile attack are made with advantage.

Wriggling Progeny (Costs 2 Actions). Each creature swallowed by the worm must succeed on a DC 20 Constitution saving throw or have its Constitution score reduced by 1d6, to a minimum of 1. A creature that has its Constitution score reduced to 1 in this way takes 28 (8d6) piercing damage and has a Century Worm Larva burrow into its body, and attach to its spine - the swallowed creature is then ejected from the century worm. 

While it has a larva burrowed into it, a creature is Charmed by the worm, and must use its action to Dash each turn until it finds soft earth in which to dig a hole and bury itself alive.