Bone Dreadnought

Bone Hopper. The Bone Dreadnought has a central section shaped like a bowl, in which spare bones are stored.  The dreadnought begins with 1000 hp worth of bones in its hopper.  These bones are used to repair damage with the dreadnought's Repair ability and with the Skeletal Catapult attack.

Whenever the Bone Dreadnought kills a swallowed creature or swallows a dead creature, it gains bones in its hopper.  Small creatures provide 5 hp worth of bones; Medium creatures provide 20 hp worth of bones; Large creatures provide 80 hp worth of bones.  The Bone Dreadnought cannot swallow creatures of Huge or larger size, but a surrounding skeleton army might butcher larger creatures and feed the pieces into the hopper, destroying the Huge or larger corpse and adding 200 hp worth of bones to the hopper after 10 minutes of work.

Repair.  The Bone Dreadnought regains 20 hp per round.  This costs 20 hp from its bone hopper.

Magic Resistance.  The Bone Dreadnought has advantage on saving throws against spells and other magical effects.

Immutable Form.  The Bone Dreadnought is immune to any spell or effect that would alter its form.

Summon Bone Devil.  If a Necromancer is driving the Dreadnought, they can use an action to summon a Bone Devil from the bones in the hopper.  This costs 50 hp from the bone hopper and can be done once per day.

Multiattack. The Bone Dreadnought makes two attacks: one with its bite and one with its skeletal catapult.  It cannot target the same creature with both attacks.

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 9) piercing damage. If the target is a Large or smaller creature, it must succeed on a DC 19 Dexterity saving throw or be swallowed by the Bone Dreadnought. A swallowed creature is restrained and it has total cover against attacks and other effects outside the dreadnought. Inside, bone saws and clawed arms try to disassemble the creature to add it to the dreadnought's hopper; the swallowed creature takes 21 (6d6) piercing damage at the start of each of the dreadnought's turns.

If the dreadnought takes 30 damage or more on a single turn from a creature inside it,  that creature can break a hole in the skeletal frame of the dreadnought and fall prone in a space within 10 feet of the dreadnought. If the dreadnought is destroyed, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement.

Skeletal Catapult.  The skeletal arm at the rear of the Bone Dreadnought reaches into the bone hopper, pulls out a giant-sized handful of bones, then flings it at a point within 100 feet.  Any creature within a 10 foot radius of the targeted point must make a DC 20 Dexterity saving throw, taking (6d6) bludgeoning damage on a failed save or half as much on a successful save.  Any structure in the radius of effect automatically takes double damage.  This action costs 10 hp from the Bone Dreadnought's bone hopper.

After hitting the ground, the bones thrown by the skeletal catapult reassemble themselves into 3 (1d6) skeleton creatures which are hostile to any non-undead creatures except the driver of the Bone Dreadnought.