Weird Aura.

At the start of each of the gargantua's turns, each creature of the gargantua's choice within 30 feet of it must make a DC 19 Wisdom saving throw. On a failed save, a target sees the gargantua as a manifestation of the target's worst fears; it takes 17 (5d6) psychic damage and has the frightened condition until the start of the target's next turn. On a successful save, a target is immune to this gargantua's Weird Aura for 24 hours.

Actions

Multiattack.

The gargantua makes two Slam or Rock attacks.

Slam.

Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 27 (3d12 + 8) bludgeoning damage.

Rock.

Ranged Weapon Attack: +15 to hit, range 120/480 ft., one target. Hit: 24 (3d10 + 8) bludgeoning damage.

Bonus Actions

Baleful Hex.

The gargantua curses one creature it can see within 120 feet of itself. The target must succeed on a DC 19 Wisdom saving throw or have the incapacitated condition until the end of its next turn.

Teleport.

The gargantua teleports, along with any equipment it is wearing or carrying, to an unoccupied space that it can see within 120 feet of itself.

Reactions

Flick.

Immediately after the gargantua takes damage from a Large or smaller creature it can see within 20 feet of itself, it attempts to flick the creature away. The target must succeed on a DC 23 Strength saving throw, or the target takes 18 (3d6 + 8) bludgeoning damage, is pushed horizontally up to 100 feet away from the gargantua, and has the prone condition.

Spell Mimicry (1/Day).

Immediately after a creature the gargantua can see casts a spell of 5th level or lower, the gargantua tries to copy the spell. That creature must succeed on a DC 19 Charisma saving throw, or the gargantua immediately casts the same spell at the same level (+11 to hit with spell attacks, spell save DC 19), requiring no material components and choosing the spell's targets.

Gargantua