Reality Warp (Mythic).

When Kar’Thuun is first reduced to 0 hit points, it does not die. Instead, it unleashes a catastrophic surge of planar distortion as its mythic trait activates. Kar’Thuun regains 300 hit points, immediately shifts into a second phase, and the area within 120 feet becomes supernaturally unstable. While in this mythic phase, Kar’Thuun’s turn includes an additional effect at initiative count 20 (losing initiative ties):

Gravity Surge. Kar’Thuun imposes a localized inversion of gravity on all other creatures within 60 feet. Each affected creature must succeed on a DC 18 Constitution saving throw or be lifted 20 feet into the air, suspended there until the start of Kar’Thuun’s next turn. A suspended creature is restrained and falls if the effect ends. This effect operates even in magical gravity or anti-gravity zones and cannot be dispelled; it is a function of reality collapsing under Kar’Thuun’s mythic presence.

Void Tendrils.

Kar’Thuun is surrounded at all times by writhing, semi-corporeal tendrils of void matter—fractures in spacetime that lash outward reflexively. Whenever a creature hits Kar’Thuun with a melee attack, that creature takes 16 (3d10) force damage as the tendrils recoil or slice through the attacker’s space. This damage ignores cover and cannot be reduced by resistance to nonmagical bludgeoning, piercing, or slashing, as it is pure extradimensional force. Kar’Thuun can suppress or reactivate the tendrils as a bonus action, though it rarely chooses to.

Phase Shift.

When Kar’Thuun is reduced to 300 hit points or fewer in its Mythic Phase, it undergoes a violent shift into an incorporeal state, becoming partially phased out of the material plane. This transformation lasts until it is destroyed or completes a long rest.

While phased:

• Kar’Thuun is incorporeal, granting it immunity to bludgeoning, piercing, and slashing damage from nonmagical attacks.

• Kar’Thuun can move through creatures and objects as if they were difficult terrain, taking 10 (3d6) force damage if it ends its turn inside an object.

• Kar’Thuun’s attack and spell effects ignore cover, passing through barriers, objects, walls, and magical fortifications so long as they do not block planar travel.

• Kar’Thuun can’t be grappled or restrained unless an effect explicitly affects incorporeal creatures.

• Its movement gains a shimmering blur; attack rolls against it have disadvantage unless the attacker has truesight.

Multiattack.

Kar’Thuun makes three Void Tendril attacks, or two Void Tendril attacks and one Void Slam.

Void Tendril. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 36 (6d10 + 10) force damage. The tendril manifests as a crackling strand of condensed void-energy that punches through matter and armor, ignoring half and three-quarters cover. If the target is a creature, its space becomes briefly distorted; the creature cannot take reactions until the start of its next turn.

Void Slam. Kar’Thuun slams a fistful of collapsing space into the ground, causing a violent gravitational implosion. All creatures of Kar’Thuun’s choice within a 20-foot radius sphere centered on a point within 60 feet must make a DC 20 Dexterity saving throw. On a failed save, a creature takes 44 (8d10) force damage, is knocked prone, and is pulled up to 10 feet toward the center of the blast. On a successful save, the creature takes half as much damage and isn’t knocked prone or pulled. This ability ignores cover as the force originates from a collapsing point in space itself.

Temporal Collapse (Recharge 5–6).

Kar’Thuun tears open a localized time-shear, causing seconds to fold violently in on themselves. Each creature of its choice within 30 feet must make a DC 20 Constitution saving throw as temporal pressure crushes their sense of motion and continuity. On a failed save, a creature takes 55 (10d10) psychic damage, becomes stunned until the start of its next turn, and its movement is halved for the following turn even after the stun ends. On a successful save, a creature takes half as much damage and its movement is halved until the end of its next turn. This effect ignores immunity to magical sleep or dream manipulation—its power is gravitational and temporal, not mental.

Tendril Strike.

Kar’Thuun makes one Void Tendril attack.

Gravity Ripple.

Kar’Thuun releases a pulse of inverted gravity.
Each creature of its choice within 15 feet must succeed on a DC 18 Strength saving throw or be knocked prone and pushed 10 feet in a direction of Kar’Thuun’s choosing.
Creatures that succeed are not moved or knocked prone but feel space flex and rumble around them.

Reality Rend (Costs 2 Actions).

Kar’Thuun twists probability around a creature it can see within 120 feet.
When a creature makes an attack roll, ability check, or saving throw, Kar’Thuun forces it to reroll.
Kar’Thuun chooses which result the creature must use.

This is not a reaction; Kar’Thuun may use it after seeing the roll result.