Ancient Platinum Dragon

Magic Master: They can't miss with their magic.

Spellcasting. The Ancient Platinum Dragon is a 20-level spellcaster. Its spellcasting ability is 50 (spell save DC 20, +0 to hit with spell attacks). The Ancient Platinum Dragon has following spells prepared:
Cantrips (at will): (Teleport), (Hyperspeed), (High jump), (Super Dash), (Lighting)
1st level (20 slots): (Grand Lighting), (Fire Storm), (Hail blizzard), (Earthquake)  2st level (10 slots): (Full Heal), (Draco Meteor), (Tunder Clap), (Hurricane)          3st level (5 slots): (Brimstone Hellfire), (Heaven Strike), (Supernova)              4st level (3 slots): (Dragon Call), (Starfall)                                                                  5st level (1 slots): (Dragon Breath)

Melee Attack:

 Tail strike: +5 to hit, reach 100 ft., lane target. Hit: 5 (10d20 + 20) All Melee damage. 

 Dash strike: +10 to hit, range 1000-200/1000 ft., area target. Hit: 5 (5d20 + 10) All Melee damage.

If the attack is not block-able than the dragon will try to dodge if it is a success than dragon take no damage. (1d20 if roll =or+15 than it is a success.)

Master of combat:

 If PCs/NPCs attack with any close-combat attack, then the Dragon will counter-attack with it tail (2d20 = melee damage).

If PCs/NPCs attack with Ranged Attack then Dragon use a 1st level spell

If PCs/NPCs use item or skip then Dragon Attack even if it not it time to attack.

block: (2d20 if roll is higher than the PCs/NPCs 1d20 roll than take no damage)

Enter the description for how legendary actions work for your monster here.

Mirror wall (Costs 1 Action) Block all magic attack for 3 round.                                                                                                                                                                Magic wall (Costs 1 Action) Block all melee attack for 3 round.            

Magic lock (Costs 1 Action) Lock ereyone but the user in place.