Balaur

Multiple Heads. The balaur has three heads. The balaur has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Wakeful. While the Balaur sleeps, at least one of its heads is awake.

Spellcasting. The balaur is a 8th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 18, +5to hit with spell attacks). The balaur has following druid spells prepared:
Cantrips (at will): Fog Cloud
3rd level ( 3 slots): Call Lightning
4th level (2 slots): Ice Storm,
7th level (1 slot): Fire Storm
8th level (1 slot): Control Weather

Multiattack. The balaur makes three bite attacks. One attack can be replaced by its tail or breath weapon if available.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 5) piercing damage plus 7 (2d6) poison damage and the target must succeed on a DC 15 Constitution saving throw against being magically petrified. On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified for 24 hours. The petrified creature becomes a gemstone with a value equal to the amount of experience points it currently has or gives in gp.

Tail. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 18 (3d8 + 5) bludgeoning damage.

Spew Stones. (Recharge 5-6). The dragon exhales a mix of poisonous gas and shards from precious stones in a 60-foot line. Each creature in that area must make a DC 18 Constitution saving throw, taking 14 (4d6) poison damage plus 14 (4d6) piercing damage on a failed save, or half as much damage on a successful one.