Pandemonium

, also known as "The Windswept Depths of Pandemonium", is a plane of madness: a great mass of rock riddled with tunnels carved by howling winds. It is cold, noisy, and dark, with no natural light. Wind quickly extinguishes non-magical open flames such as torches and campfires. It also makes conversation possible only by yelling, and even then only at a maximum distance of ten feet. Most of the plane's inhabitants are creatures that were banished to the plane with no hope of escape, and many of them have been driven mad by the incessant winds or forced to take shelter in places where the winds die down until they sound like distant cries of torment.

The Search for Grog
Vox Machina traveled to this plane to retrieve Grog's soul after he drew the "Void" card from the Deck of Many Things. The soul was trapped in a glowing red gem atop the Howler's Crag in Cocytus (the second layer of Pandemonium). The gem was being guarded by a corrupted empyrean, which was slain by Grog after his soul was restored to his body.

The Search for Bob
With Grog's soul restored to his body and the corrupted empyrean slain, Vox Machina then set about recovering their stolen spell components from the thieving githzerai known to the party as "Bob".

Inhabitants

 * "Bob/Shanak"
 * Howlers
 * Balor
 * Bugbears
 * "The Roiling Anger" (a corrupted empyrean)
 * "The Rolling Thunder" (a dust titan)

Trivia

 * Matt revealed in a quick epilogue that the empyrean was the son of the former god of death...the now-forgotten deity usurped by the Matron of Ravens after she ascended to godhood. The empyrean had been imprisoned on Pandemonium over a millennium ago as a short-term punishment.  But with the death of his father, the empyrean was never retrieved and became corrupted.  Unaware of his father's demise, the empyrean had become insane...begging for death and for mercy from a father long since destroyed.