Adult Magma Dragon

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

Scorching Aura. When a creature comes within 10ft of the dragon for the first time on its turn or starts its turn there, it must succeed on a DC 22 Constitution saving throw or take 27(5d10) fire damage. Once the creature comes within 5ft of the dragon or if it starts its turn there, the creature takes the damage if it hasn't already, regardless if whether or not it succeeded on the saving throw. 

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

Magic Weapons. The dragon's weapon attacks are magical. 

Multiattack: The dragon makes three attacks, one with its Bite and two with its Claws.

Bite: Melee weapon attack, +14 to hit, reach 10ft, one target. Hit 11(2d6+8) piercing damage and the target must succeed on a DC 22 Dexterity saving throw or be swallowed by the dragon. A swallowed creature is Blinded and Restrained, has total cover against attacks and other effects outside the dragon, and has its hit point maximum reduced by 5 at the start of each of the dragon's turns. This reduction lasts until the target finishes a long rest. 

If the dragon takes 35 damage or more on a single turn from a swallowed creature, the dragon must succeed on a DC 23 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall Prone in a space within 10ft of the dragon. If the dragon dies, a swallowed creature is no longer Restrained and can escape the corpse using 20ft of movement, exiting Prone. 

Claw: Melee weapon attack, +14 to hit, reach 5ft, one target. Hit 10(1d4+8) slashing damage. If the target is a medium or smaller creature, it is grappled(escape DC 18). Until this grapple ends, the target is Restrained, and the dragon can't grapple another target. 

Tail: Melee weapon attack, +14 to hit, reach 15ft, one target. Hit 19(2d10+8) bludgeoning damage and the target must succeed on a DC 22 Strength saving throw or be knocked Prone. 

Lava Breath(Recharge 5-6): The Lava Dragon exhales lava in a 60ft cone. Each creature in that area must make a DC 22 Dexterity saving throw, taking 14(4d6) bludgeoning damage plus 27(5d10) fire damage on a failed save, and half as much damage on a successful one. A creature that failed the saving throw is buried under hot and heavy rock until it is freed. A buried creature is Restrained and takes 7(2d6) bludgeoning damage plus 27(5d10) fire damage at the start of each of its turns. A creature can use its action to make a DC 18 Strength check, freeing itself or another creature within 5ft of it on a success. 

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn:

Detect: The dragon makes a Wisdom(Perception) check. 

Tail Attack: The Dragon makes a Tail attack. 

Wing Attack(Cost 2 Actions): The dragon beats its wings, spraying molten rock around it. Each creature within 10ft of it must make a DC 22 Dexterity saving throw, taking 27(5d10) fire damage on a failed save, and half as much damage on a success. Additionally, if the creature failed the saving throw, it is knocked Prone. The dragon can then fly up to half its flying speed.