Legendary Resistance (3/Day). If the Centipede fails a saving throw, it can choose to succeed instead.

Freedom of movement. The Centipede ignores difficult terrain, and magical effects can't reduce its speed or cause it to be restrained. It can spend 5 feet of movement to escape from non-magical restraints or being grappled.

Siege Monster. The Centipede deals double damage to objects and structures.

Multipart. The Centipede consists of 4 bound together Large size parts - head, 2 body parts and tail. Each part have 10d20 + 25hp (130hp). After defeating each part, Centipede lose some of its abilities: 

• Head. After defeating head, Centipede loses its ability of spell casting.
• Body. After defeating each body parts, Centipede loses grapple from its attacks.
• Tail. After defeating tail, Centipede loses tail slam from its attack

Second wind. After defeating all parts, Centipede breaks down to separate 4 parts. Each part have 5d20 + 30hp (83 hp). Each part now can move separately from each other. Also, each part gain new mythic abilities

Spellcasting (Head only). The Centipede is an 9 level spellcaster. Its spellcasting ability is Wisdom (spell save DC 18, +10 to hit with spell attacks). The Centipede has the following spells prepared:
Cantrips (at will): Toll the Dead, Mind Sliver, Minor Illusion, Friends
1st level (4 slots):  Charm Person,  Absorb Elements, Cause fear, Shield
2nd level (3 slots): Tasha's mind whip, Hold person, Ray of enfeeblement, Darkness
3rd level (3 slots): Hypnotic pattern, Counterspell, Fear
4th level (3 slots): Raulothim's Psychic Lance, Evard's black tentacles
5th level (1 slots): Cloudkill

Grapple (Body only). Melee Weapon Attack: +8 to hit, reach 10 ft., 1 target. Hit: 19 (4d6 + 5) bludgeoning damage, and the target is grappled (escape DC 16) if it is a Huge or smaller creature

Slam (Tail only). The Centipede slams its tail of the ground. Each creature in a 25-ft cube around the tail makes a Constitution saving throw (DC 18). On a failed saving throw, each target takes 3d8 thunder damage and pushed 10 feet away from tail. On a successful save, the creature takes half as much damage and isn’t pushed.

The Centipede can take 4 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. Centipede regains spent legendary actions at the start of its turn.

Cantrip. Centipede's head casts a cantrip.

Grapple. Centipede's body uses a grapple action

Slam. Centipede's tail uses a slam action

Move (Costs 2 Actions). Centipede moves

After defeating each of the Centipede's parts, each part starts moving separately. Each part has 5d20 + 30 hp (83 hp), loses Legendary Actions, and gains Mythic Action.

Part can take 4 mythic actions, choosing from the options below. All parts use the same pool of actions. Only one mythic action option can be used at a time, and only at the end of another creature’s turn. Part regains spent mythic actions at the start of its turn.

Acid splash. Part spews a big blob of acid in a line 50 feet long and 5 feet wide. Each creature in the line must make a DC 16 Dexterity saving throw, taking 14 (4d6) acid damage on a failed save, or half as much damage on a successful one.