Adult glass dragon

 Magic Resistance. The dragon has advantage on saving throws against spells and other magical effects. Unless the spell does thunder damage 

Reflective Carapace. Any time the dragon is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the dragon is unaffected. On a 6, the dragon is unaffected, and the effect is reflected back at the caster as though it originated from the dragon, turning the caster into the target. This automatically happens if its magic missile or any spell that gives the blinded condition or creates light 

 
Limited Magic Immunity. The dragon can't be affected or detected by spells of 1st level or lower unless the spell does thunder damage 

Clear Scales: Each creature that has not already spotted the dragon and is searching needs to make a DC 15 wisdom perception check to spot the dragon as its translucent scales bend light around it. (If a player loses sight of the dragon at the end of its turn the check must be re-made or attacks have disadvantage when targeting the glass dragon.)

Sharded Shape. As the dragon is beaten and battered the glass of its scales begin to shatter and break , once the dragon reaches below 50% of its maximum HP It losses the clear scales trait but gains ever time it gets hit by a attack all creatures in 5ft radius around it takes 2d6 piercing damage from glass shards

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

Spellcasting. The dragon is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 20, +12 to hit with spell attacks). The lich has the following wizard spells prepared:

Cantrips (at will): fire bolt, minor illusion

1st level (5 slots): detect magic, magic missile, color spray 

2nd level (5 slots): detect thoughts, Invisibility, mirror image

3rd level (4 slots): counterspell, dispel magic, blink,Glyph of Warding

4th level (2 slots): Greater Invisibility, Leomund's Secret Chest

5th level (2 slots): wall of force,

6th level (1 slot): True Seeing, scrying 

7th level (2 slot): planar shift,Etherealness

8th level (1 slot): maze

9th level (1 slot): astral projection,

Multiattack. The dragon can use its Alluring light. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 4 (1d8) slashing damage.

Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Breath Weapons (Recharge 5–6). The dragon uses one of its breath attacks 

Molten Breath. The dragon spews a molten blast in a 35-foot line 5ft wide or a 25ft cone Each creature in that area must make a DC 20 Dexterity saving throw, taking 58 (13d8) fire damage on a failed save and is restrained as the molten glass begins to harden, or half as much damage on a successful one and is not restrained but has its movement speed reduced by 15ft

Shard breath. The dragon exhales hail of shards of glass in a 60-foot cone. Each creature in that area must succeed on a DC 20 Constitution saving throw taking 35 (8d8) piercing damage and 35 (8d8) slashing damage

frenzy when the dragon is below half health it can do a claw attack as a bonus action 

The dragon 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. 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.

glassy gaze(Costs 1 Actions): The dragon targets a creature it can see within 60 feet. The creature must succeed on a dc 15 Wisdom saving throw or be stunned until the end of its next turn as the dragon's gaze momentarily paralyzes it.

Shattering shard (Costs 1 Actions) the dragon can launch a shard of its body at a target. The shard deals 1d6 piercing damage and the creature needs to make a dc 16 Constitution saving throw, or the target is vulnerable to slashing damage for 1 minute 

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 19 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.