Chymedra

Way Too Many Heads. The hydra has at least five heads, and can't have less heads than this unless severed by an effect such as a Vorpal Sword or Psychic Scream describes otherwise.

The Chymedra starts with the MBT-808 cannon attached to its back, and the following heads;
1. Wild Mage*
8. Owlbear
11. Giant Shark
15. Beholder*
17. Adult Red Dragon.*

The Chymedra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Whenever the hydra takes 25 or more damage in a single turn, one of its heads dies. At the end of that turn, the Chymedra regrows 2 heads for each multiple of 25 taken. The Chymedra heals 25 hit points per 2 heads regrown.

If an effect would stop the Chymedra from regaining hit points, it instead stops the regeneration of one set of heads.

Reservoir of Meat. The Chymedra has way too much meat packed into its form. Whenever the Chymedra rolls to regenerate a head, roll a d20. Reroll if you already have the Wild Mage or Adult Red Dragon head.

Reactive Heads. For each head the Chymedra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Multiattack. The Chymedra makes as many attacks as it has heads, and an MBT-808: Main Cannon attack, if loaded.

MBT-808: Main Cannon. Ranged Weapon Attack: range 300/900 ft. The MBT-808 fires a large explosive round at a point on the ground or other physical surface. Creatures in that area must make a DC 21 Dexterity Saving throw, dealing 20d6 piercing and 20d6 bludgeoning on a failed save, or half as much on a success.

1. Wild Mage: Surge of Power. Uses the Wand of Wonder. In addition, any creature within sight of the Wild Mage can accept a radiant Healing Word, but must immediately roll on the Wild Magic Surge table.

2. Goat: Horns. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 3 (1d4 + 1) bludgeoning damage.

3. Zombie: Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.

4. Guard: Spear. Melee or Ranged Weapon Attack: +3 to hit, reach 5 ft. or range 20/60 ft., one target. Hit:  5 (1d8 + 1) piercing damage.

5. Drow: Poison Crossbow. Ranged Weapon Attack: +4 to hit, range 30/120 ft., one target. Hit: 5 (1d6 + 2) piercing damage, and the target must succeed on a DC 13 Constitution saving throw or be poisoned for 1 hour. If the saving throw fails by 5 or more, the target is also unconscious while poisoned in this way. The target wakes up if it takes damage or if another creature takes an action to shake it awake.

6. Rug of Smothering: Smother. Melee Weapon Attack: +5 to hit, reach 5 ft., one Medium or smaller creature. Hit: The creature is grappled (escape DC 13). Until this grapple ends, the target is restrained, blinded, and at risk of suffocating, and the rug can't smother another target. In addition, at the start of each of the target's turns, the target takes 10 (2d6 + 3) bludgeoning damage.

7. Gelatinous Cube: Engulf. Melee Weapon Attack: One Large or smaller creature within 5 ft. Creature must make a DC 12 Dexterity saving throw. On a failed save, the cube enters the creature's space, and the creature takes 10 (3d6) acid damage and is engulfed. The engulfed creature can't breathe, is restrained, and takes 21 (6d6) acid damage at the start of each of the cube's turns. When the cube moves, the engulfed creature moves with it.

An engulfed creature can try to escape by taking an action to make a DC 12 Strength check. On a success, the creature escapes and enters a space of its choice within 5 feet of the cube.

8. Owlbear: Peck. Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 10 (1d10 + 5) piercing damage.

9. Unicorn: Impale. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage.

10. Mimic: Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: 10 (1d10 + 5) piercing damage plus 7 (2d6) acid damage.

11. Giant Shark: Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 22 (3d10 + 6) piercing damage.

12. Mind Flayer: Tentacles/Extract Brain. Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 15 (2d10 + 4) psychic damage. If the target is Medium or smaller, it is grappled (escape DC 15) and must succeed on a DC 15 Intelligence saving throw or be stunned until this grapple ends.

13. Blue Slaad: Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 5) slashing damage. If the target is a humanoid, it must succeed on a DC 15 Constitution saving throw or be infected with a disease called chaos phage. While infected, the target can't regain hit points, and its hit point maximum is reduced by 10 (3d6) every 24 hours. If the disease reduces the target's hit point maximum to 0, the target instantly transforms into a red slaad or, if it has the ability to cast spells of 3rd level or higher, a green slaad. Only a wish spell can reverse the transformation.

14. Roc: Beak. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 27 (4d8 + 9) piercing damage.

15. Beholder: Eye Ray. The beholder shoots a random magical eye rays at each target it can see within 120 feet.

16. Vampire: Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one willing creature, or a creature that is grappled by the vampire, incapacitated, or restrained. Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0. A humanoid slain in this way and then buried in the ground rises the following night as a vampire spawn under the vampire's control.

17. Adult Red Dragon: Fire Breath (3/day) The dragon exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one.

18. Balor: Fire Whip. Melee Weapon Attack: +14 to hit, reach 30 ft., one target. Hit: 15 (2d6 + 8) slashing damage plus 10 (3d6) fire damage, and the target must succeed on a DC 20 Strength saving throw or be pulled up to 25 feet toward the balor.

19. Lich: Paralyzing Touch. Melee Spell Attack: +12 to hit, reach 5 ft., one creature. Hit: 10 (3d6) cold damage. The target must succeed on a DC 18 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

20. Marut: Unerring Slam. Melee Weapon Attack: automatic hit, reach 5 ft., one target. Hit: 60 force damage, and the target is pushed up to 5 feet away from the marut if it is Huge or smaller.

READY. (1/Round) The MBT-808 loads a round into the Main Cannon.

AIM. (1/Round) The MBT-808 aims the Main Cannon in a 45 degree cone.

MYTHIC ACTION: Eye Ray. At the end of the current creature's turn, The Beholder uses a random eye ray against that creature if they are within 120 feet.