Reverberation Affected entity has a -1 penalty to Strength, Dexterity, and Constitution 
Saving throws per remaining turn.
When the entity has 5 or more turns of Reverberation, it takes 1~4 Thunder damage and must succeed a DC 10 Constitution saving throw or fall Prone. The condition is removed afterward.
Creatures Immune to 
Thunder damage can't recieve Reverberation.
Sources of Reverberation
Notes
Reverberation does apply the stacking penalties to Strength, Dexterity, and Constitution Saving Throws. As of Patch #3, Reverberation is no longer bugged. The effect at 5 stacks which causes thunder damage and possibly knocks the enemy prone now works as described.
The different items that can apply Reverberation do not all work as expected from their description.
Items that Apply Reverberation
Gloves of Belligerent Skies: When the wielder deals 
Radiant, 
Lightning, or Thunder damage, apply 2 turns of Reverberation to the target(s).
Boots of Stormy Clamour: When the wearer inflicts a condition upon a hostile creature, they also inflict 2 turns of Reverberation.
Spineshudder Amulet: When the wearer deals damage with a ranged Spell Attack, inflict 2 turns of 
on the target(s).
Markoheshkir: Grants 
which can grant 
: When you deal spell damage, inflict 1 turn of Reverberation upon the target. [As well as other benefits]
Items that Interact with Reverberation
Ring of Absolute Force: If the wearer bears the Absolute's brand, they deal 1 additional Thunder damage. NB : this should work with reverberation, but can't be tested as reverberation doesn't work for now.
Thunderskin Cloak: When a creature with deals damage to the wearer, the creature needs to make a Constitution Saving Throw or become 
.
Ring of Spiteful Thunder: When the wearer deals Thunder damage to a creature, it becomes unless it succeeds a Constitution Saving Throw.
