When the hazard triggers, creatures in the affected area must make a DC 16 Dexterity saving throw. On a failed save, a creature takes 22 (4d10) bludgeoning damage, is knocked prone, and is buried under rubble. On a successful save, it takes half damage and is not knocked prone. A buried creature is restrained and blinded, has total cover from creatures outside the rubble, and can use its action to make a DC 16 Strength (Athletics) check, freeing itself on a success. Other creatures can free it by dealing 20 damage to the rubble, which has AC 15 and immunity to psychic and poison damage.

After the initial collapse, the ravine remains unstable for 1 minute. At the end of each creature's turn while it is in the affected area, it must succeed on a DC 13 Dexterity saving throw or take 5 (1d10) bludgeoning damage from secondary debris. A creature that moves at half speed through the area has advantage on this saving throw.

The trigger point is a 30-foot-long section of ravine beneath an unstable shelf. Creatures in the section when the collapse begins are affected. Creatures within 10 feet of either end can use their reaction to move up to 10 feet if they can see a safe space and succeed on a DC 14 Dexterity (Acrobatics) check.

Thunder damage, explosions, siege weapons, a creature falling against the cliff, or a failed check to climb the ravine can trigger the hazard. The DM may also trigger it when three or more creatures move at full speed through the same unstable section.

A creature that succeeds on the initial saving throw can choose to cling to the ravine wall instead of remaining on the ground. Until the end of its next turn, it must succeed on a DC 14 Strength (Athletics) check or fall prone as the debris shifts beneath it.