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

• Pools of water that Yvrenoth can see within 120 feet of it surge outward in a grasping tide. Any creature on the ground within 20 feet of such a pool must succeed on a DC 15 Strength saving throw or be pulled up to 20 feet into the water and knocked prone.
• A cloud of swarming insects fills a 20-foot-radius sphere centered on a point Yvrenoth chooses within 120 feet of it. The cloud spreads around corners and remains until the dragon dismisses it as an action, uses this lair action again, or dies. The cloud is lightly obscured. Any creature in the cloud when it appears must make on a DC 15 Constitution saving throw, taking 10 (3d6) piercing damage on a failed save, or half as much damage on a successful one. A creature that ends its turn in the cloud takes 10 (3d6) piercing damage.
• Magical darkness spreads from a point Yvrenoth chooses within 60 feet of it, filling a 15-foot-radius sphere until the dragon dismisses it as an action, uses this lair action again, or dies. The darkness spreads around corners. A creature with [Tooltip Not Found] can’t see through this darkness, and nonmagical light can’t illuminate it. If any of the effect’s area overlaps with an area of light created by a spell of 2nd level or lower, the spell that created the light is dispelled.

• Part of the ceiling collapses above one creature that Yvrenoth can see within 120 feet of it. The creature must succeed on a DC 15 Dexterity saving throw or take 10 (3d6) bludgeoning damage and be knocked prone and buried. The buried target is restrained and unable to breathe or stand up. A creature can take an action to make a DC 10 Strength check, ending the buried state on a success.
• A cloud of sand swirls about in a 20-foot-radius sphere centered on a point Yvrenoth can see within 120 feet of it. The cloud spreads around corners. Each creature in the cloud must succeed on a DC 15 Constitution saving throw or be blinded 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.
• Lightning arcs, forming a 5-foot-wide line between two of the lair’s solid surfaces that Yvrenoth can see. They must be within 120 feet of the dragon and 120 feet of each other. Each creature in that line must succeed on a DC 15 Dexterity saving throw or take 10 (3d6) lightning damage.

• Grasping roots and vines erupt in a 20-foot radius centered on a point on the ground that Yvrenoth can see within 120 feet of it. That area becomes difficult terrain, and each creature there must succeed on a DC 15 Strength saving throw or be restrained by the roots and vines. A creature can be freed if it or another creature takes an action to make a DC 15 Strength check and succeeds. The roots and vines wilt away when the dragon uses this lair action again or when the dragon dies.
• A wall of tangled brush bristling with thorns springs into existence on a solid surface within 120 feet of Yvrenoth. The wall is up to 60 feet long, 10 feet high, and 5 feet thick, and it blocks line of sight. When the wall appears, each creature in its area must make a DC 15 Dexterity saving throw. A creature that fails the save takes 18 (4d8) piercing damage and is pushed 5 feet out of the wall’s space, appearing on whichever side of the wall it wants. A creature can move through the wall, albeit slowly and painfully. For every 1 foot a creature travels through the wall, it must spend 4 feet of movement. Furthermore, a creature in the wall’s space must make a DC 15 Dexterity saving throw once each round it’s in contact with the wall, taking 18 (4d8) piercing damage on a failed save, or half as much damage on a successful one. Each 10-foot section of wall has AC 5, 15 hit points, vulnerability to fire damage, resistance to bludgeoning and piercing damage, and immunity to psychic damage. The wall sinks back into the ground when the dragon uses this lair action again or when the dragon dies.
• Magical fog billows around one creature Yvrenoth can see within 120 feet of it. The creature must succeed on a DC 15 Wisdom saving throw or be charmed by the dragon until initiative count 20 on the next round.

• Magma erupts from a point on the ground Yvrenoth can see within 120 feet of it, creating a 20-foot-high, 5-foot-radius geyser. Each creature in the geyser’s area must make a DC 15 Dexterity saving throw, taking 21 (6d6) fire damage on a failed save, or half as much damage on a successful one.
• A tremor shakes the lair in a 60-foot radius around the dragon. Each creature other than Yvrenoth on the ground in that area must succeed on a DC 15 Dexterity saving throw or be knocked prone.
• Volcanic gases form a cloud in a 20-foot-radius sphere centered on a point Yvrenoth can see within 120 feet of it. The sphere spreads around corners, and its area is lightly obscured. It lasts until initiative count 20 on the next round. Each creature that starts its turn in the cloud must succeed on a DC 13 Constitution saving throw or be poisoned until the end of its turn. While poisoned in this way, a creature is incapacitated.

• Freezing fog fills a 20-foot-radius sphere centered on a point Yvrenoth can see within 120 feet of it. The fog spreads around corners, and its area is heavily obscured. Each creature in the fog when it appears must make a DC 10 Constitution saving throw, taking 10 (3d6) cold damage on a failed save, or half as much damage on a successful one. A creature that ends its turn in the fog takes 10 (3d6) cold damage. A wind of at least 20 miles per hour disperses the fog. The fog otherwise lasts until the dragon uses this lair action again or until the dragon dies.
• Jagged ice shards fall from the ceiling, striking up to three creatures underneath that Yvrenoth can see within 120 feet of it. The dragon makes one ranged attack roll (+7 to hit) against each target. On a hit, the target takes 10 (3d6) piercing damage.
• Yvrenoth creates an opaque wall of ice on a solid surface it can see within 120 feet of it. The wall can be up to 30 feet long, 30 feet high, and 1 foot thick. When the wall appears, each creature within its area is pushed 5 feet out of the wall’s space, appearing on whichever side of the wall it wants. Each 10-foot section of the wall has AC 5, 30 hit points, vulnerability to fire damage, and immunity to acid, cold, necrotic, poison, and psychic damage. The wall disappears when the dragon uses this lair action again or when the dragon dies.

• A strong wind blows around Yvrenoth. Each creature within 60 feet of the dragon must succeed on a DC 15 Strength saving throw or be pushed 15 feet away from the dragon and knocked prone. Gases and vapors are dispersed by the wind, and unprotected flames are extinguished. Protected flames, such as lanterns, have a 50 percent chance of being extinguished.
• A cloud of sand swirls about in a 20-foot-radius sphere centered on a point Yvrenoth can see within 120 feet of it. The cloud spreads around corners. Each creature in it must succeed on a DC 15 Constitution saving throw or be blinded 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.

• Yvrenoth creates fog as though it had cast the fog cloud spell. The fog lasts until initiative count 20 on the next round.
• A thunderclap originates at a point Yvrenoth can see within 120 feet of it. Each creature within a 20-foot radius centered on that point must make a DC 15 Constitution saving throw or take 5 (1d10) thunder damage and be deafened until the end of its next turn.

• Yvrenoth chooses a point on the ground that it can see within 120 feet of it. Stone spikes sprout from the ground in a 20-foot radius centered on that point. The effect is otherwise identical to the spike growth spell and lasts until the dragon uses this lair action again or until the dragon dies.
• Yvrenoth chooses a 10-foot-square area on the ground that it can see within 120 feet of it. The ground in that area turns into 3-foot-deep mud. Each creature on the ground in that area when the mud appears must succeed on a DC 15 Dexterity saving throw or sink into the mud and become restrained. A creature can take an action to attempt a DC 15 Strength check, freeing itself or another creature within its reach and ending the restrained condition on a success. Moving 1 foot in the mud costs 2 feet of movement. On initiative count 20 on the next round, the mud hardens, and the Strength DC to work free increases to 20.

• Yvrenoth glimpses the future, so it has advantage on attack rolls, ability checks, and saving throws until initiative count 20 on the next round.
• One creature Yvrenoth can see within 120 feet of it must succeed on a DC 15 Charisma saving throw or be banished to a dream plane, a different plane of existence the dragon has imagined into being. To escape, the creature must use its action to make a Charisma check contested by the dragon’s. If the creature wins, it escapes the dream plane. Otherwise, the effect ends on initiative count 20 on the next round. When the effect ends, the creature reappears in the space it left or in the nearest unoccupied space if that one is occupied.

• Yvrenoth creates fog as if it had cast the fog cloud spell. The fog lasts until initiative count 20 on the next round.
• A blisteringly cold wind blows through the lair near Yvrenoth. Each creature within 120 feet of the dragon must succeed on a DC 15 Constitution saving throw or take 5 (1d10) cold damage. Gases and vapors are dispersed by the wind, and unprotected flames are extinguished. Protected flames, such as lanterns, have a 50 percent chance of being extinguished.