Ancient Seer Dragon

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

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

Cantrips (at will): vicious mockery, mage hand

1st level (4 slots): shield, magic missile, sleep

2nd level (3 slots): detect thoughts, heat metal, suggestion

3rd level (3 slots): bestow curse, dispel magic, major image

4th level (3 slots): greater invisibility, hallucinatory terrain

5th level (3 slots): animate objects, dominate person

6th level (1 slot): eyebite, trueseeing

7th level (1 slot): regenerate, symbol

8th level (1 slot): feeblemind, power word stun

9th level (1 slot): foresight

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

Bite. Melee Weapon Attack: + 15 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage. 

Claw. Melee Weapon Attack: +15 to hit, range 10 ft., one target. Hit: 22 (4d6 + 8) slashing damage. 

Tail. Melee Weapon Attack: +15 to hit, range 20 ft., one target. Hit:  17 (2d8 + 8) bludgeoning damage.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 19 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

Infect Mind (Recharge 5-6). The Seer Dragon can attempt to look into the mind of a creature, thereby infecting them and rendering them helpless. As an action, one target that the Seer Dragon can see must make an Intelligence saving throw or take 10d8 psychic damage and have their Intelligence score reduced by 2 until the end of a short or long rest. If a target's Intelligence score is reduced to 0, they are unconscious and dying. This effect can be reversed by any healing spell such as Spare The Dying or Cure Wounds. If this is cast on a creature, they regain one Intelligence and that many number of hit points. All lost Intelligence scores are regained after a short or long rest.

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.

Investigate (Costs 1 Action). The dragon makes an Intelligence (Investigation) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack. (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 23 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.