Astrum Deus

Astral Duality. If Astrum deus is reduced to 115 hit points of fewer, he turns invisible and invincible for 1 round, then re-appears as two worms, each with 115 hit points. Each has 36 segments and lacks the Legendary Resistance trait but otherwise retains the same statistics as the original worm. The small worms can also use the same attacks as the main worm. If one of the worms is killed, the other worm also dies, due to both being psychically connected to each other.

Astral Worm. Astrum Deus is made up of 71 Medium segments: one head segment, 69 body segments, and one tail segment. Only the head segment can make bite attacks, only the body segments can make slam and Astral Laser Barrage attacks, and only the tail can make tail attacks and Astral Mine attacks.

Astral Infection. A creature who hits Astrum Deus with a melee attack must succeed on a DC 18 CON save or become temporarily infected with an astral disease. The creature has disadvantage on attack rolls and takes 10(4d4) necrotic damage on its next turn.

Magic Resistance. Astrum Deus has advantage on all saving throws against magical effects and spells.

Legendary Resistance(1/day). If Astrum Deus fails a saving throw, it can choose to succeed instead.

Magic Weapons. Astrum Deus's weapon attacks are magical.

Earth Glide. Astrum Deus can move through solid surfaces such as walls and ceilings at its burrowing speed without disturbing the material it moves through. 

Bite. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 41(6d10 + 8) piercing damage, and the target must succeed on a DC 18 CON save or become infected with an astral disease for 1 minute. The target takes 7(3d4) necrotic damage at the start of each of its turns. The target can repeat the saving throw at the end of each of its turns, ending the effect on a success.

Body Slam. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 26(4d8 + 8) bludgeoning damage.

Tail. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 21(3d8 + 8) bludgeoning damage.

Astral Laser Spread(Recharge 5-6). Astrum Deus sends a barrage of astral lasers from its body segments. All creatures on one side of the worm(the DM chooses which side) must succeed on a DC 17 DEX save or take 22(5d8) necrotic damage.

Astral Mine. Astrum Deus drops astral mines from its tail. A mine is invincible and takes up a 5 ft area. A creature that moves into that area must succeed on a DC 17 DEX save or take 18(4d8) bludgeoning danage and 18(4d8) necrotic damage.

Astrum Deus 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. Astrum Deus regains spent legendary actions at the start of its turn.

Move. Astrum Deus moves up to half its speed without provoking any oppurtunity attacks.

Astral Laser Spread(Costs 3 actions). Astrum Deus recharges and uses Astral Laser Spread.

Dead God's Regeneration(Costs 3 Actions, before splitting). Astrum Deus regains 4d6+12 hit points.