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

• The ceiling collapses above one creature that the dragon can see within 120 feet of it. The creature takes 10 (3d6) bludgeoning damage and is knocked prone and restrained (by fallen debris); damage is halved and the creature is not restrained if it makes a successful DC 15 Dexterity saving throw. The creature is freed when it or an adjacent ally uses an action to make a successful DC 15 Strength (Athletics) check.

• A ten foot-wide, ten foot-long crack opens in the cavern floor where the dragon wishes. Any creature occupying that space must make a successful DC 15 Dexterity saving throw or fall 20 feet, taking 7 (2d6) bludgeoning damage plus 7 (3d4) piercing damage from the jagged stones at the bottom.

• The dragon summons a swarm of insects as if it had cast insect plague, filling a 20-foot radius sphere within 90 feet of the dragon. Creatures that are in the affected space or that enter it take 22 (4d10) piercing damage, or half damage with a successful DC 18 Constitution saving throw. The swarm lasts until initiative count 20 on the next round.

An adult or older cave dragon can generate an aura of darkness that fills its space and the surrounding 20 feet. This darkness prevents normal vision and darkvision from functioning. Blindsight and truesight function normally. Activating or deactivating the aura is a bonus action.

An adult cave dragon glides through stone, dirt, or any sort of earth except metal as easily as a fish glides through water. Its burrowing produces no ripple or other sign of its presence and leaves no tunnel or hole unless the dragon chooses to do so; in that case, it creates a passageway 15 feet wide by 10 feet high. The spell move earth cast on an area containing an earth-gliding cave dragon flings the dragon back 30 feet and stuns the creature for one round unless it succeeds on a Constitution saving throw.

If the dragon fails a saving throw, it can choose to succeed instead.

the region containing a legendary cave dragon’s lair is warped by the dragon’s magic, which creates one or more of the following effects:

• Poisonous and odorless gases suddenly fill passages and caverns, and just as quickly disperse, within six miles of the dragon’s lair.

• Flash flooding turns tunnels into death traps as tremors create fissures in the stone within six miles of the lair. On the surface, ponds drain away, and long-dry creek beds break their banks in flood.

• Swarms of vermin within one mile of the lair increase in both size and number as they try to escape the dragon’s endless and undiscriminating hunger.

if the dragon dies, these effects fade over the course of 1d10 days.