Actions

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: +19 to hit, reach 3 sq., one target. Hit: 31 ( 4d10+10 ) piercing damage.   Claw. Melee Weapon Attack: +19 to hit, reach 2 sq., one target. Hit: 25 ( 3d8+10 ) slashing damage.   Tail. Melee Weapon Attack: +19 to hit, reach 4 sq., one target. Hit: 27 ( 3d10+10 ) bludgeoning damage.   Frightful Presence. Each creature of dragon's choice within 24 square of it and aware of it must succeed on a DC 26 Wisdom saving throw or become frightened for 1 minute. A frightened target can repeat the saving throw at the end of each of its turns, with disadvantage if the dragon is within line of sight, ending the effect on itself on a success.   Breath Weapons (Recharge 1d6>=5 ). The dragon uses one of the following breath weapons:

• Fire Breath. The dragon exhales fire in a 18-square cone. Each creature in that area must make a DC 26 Dexterity saving throw, taking 91 ( 26d6 ) fire damage on a failed save, or half as much damage on a successful one.
• Psychic Breath. The dragon exhales a blast of psychic energy in a 12-square cone. Each creature in that area must make a DC 26 Intelligence saving throw, taking 84 ( 13d12 ) psychic damage on a failed save, or half as much damage on a successful one.

Legendary Actions

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.
• Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 3 square of it must succeed on a DC 27 Dexterity saving throw or take 17 ( 2d6+10 ) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Lair Actions

On initiative count 20 (losing initiative ties), the dragon can take a lair action to cause one of the following effects; the dragon can't use the same effect two rounds in a row:  

• Arcane Overflow. Magical energy floods a 4-square radius sphere centered on a point the dragon can see within 24 squares. Each creature in the area must succeed on a DC 26 Intelligence saving throw or take 28 ( 8d6 ) force damage and be stunned until the end of their next turn.
• Knowledge Drain. The dragon targets a creature it can see within 24 squares. The target must succeed on a DC 26 Wisdom saving throw or have disadvantage on Intelligence and Wisdom saving throws until the end of its next turn.
• Library's Defense. The dragon conjures a shield of magical books. Until initiative count 20 on the next round, the dragon has three-quarters cover against all attacks and effects.

Regional Effects

The region containing the dragon's lair is warped by its magic, creating the following effects:  

• Altered Perceptions. Non-magical reading materials within 6 miles of the lair are filled with shifting text and illusions, making them difficult to read.
• Arcane Echoes. Spells cast within 1 mile of the lair echo with additional effects. When a creature casts a spell of 1st level or higher within this area, roll a d10 . On a 1, the creature must roll on the Wild Magic Surge table in the Player's Handbook.
• Mental Overload. Creatures that finish a long rest within 6 miles of the lair must succeed on a DC 15 Intelligence saving throw or be plagued by unnerving dreams that cause them to gain no benefit from that rest.