Gatshadow Mountain

 was an enormous, ominous mountain in Tal'Dorei at the southeastern tip of the Cliffkeep Mountains, just northwest of the Bramblewood Forest and Westruun. It got its name because it was dark and ominous with a dark, blackened rock peak that stood out amongst the rest of the mountains. While other mountains crest at 4,000 feet up, this one was over a mile high. Gatshadow's peak was nearly always obscured by storm clouds or mist. Most of the trees were dead or dried out towards the top, but other than that the vegetation was pretty sparse.

The actual stone that comprised this monstrous formation was of a darker, more dense stone and it was considered to be the finest stone for crafting on the Tal'Dorei continent, hence why many crafters and smiths made the dangerous journey there to quarry the rock—and not all return.

Entrance
Around the big cavern entrance there are clumps of dead, petrified trees. Leafless, gnarled, and devoid of any life on top off melted stone and soot. The entrance is just an opening on the side of the mountain with no walkable path up to it. 60 feet below is a smaller entrance with a 40 foot scalable wall leading towards it at an upward angle.

Around the exterior of the entrance, there are two rocks embedded in each side of the cavern wall, about three to four foot in diameter that are nearly identical. Their texture and their weathered appearance is far younger than the rest of the stone around them.

Small Entrance
This hole continued for 15 feet into the mountain before it ended abruptly by a cave in. A layer of dust sits on everything and other than the occasional gust of wind that finds its way in here, it is largely undisturbed for a long period of time.

Cavern
In the cavern there are pieces of stone jutting out of the floor, two pools of somewhat greenish-brackish pungent-smelling acid. Next to one of the pools there is an elevated platform where a large outcropping of stone has just slowly pulled itself out of the rock, nearly touching the very height of this cavern. The ceiling is 70 feet high with a few stalactites points of rock throughout this cavern.

There are portion of the mineral growth, and a lot of the moisture that have coalesced into the ceiling come into a central point, and over time, it has built up central columns of rock and natural materials. A few pools gathered towards the base. Portions of the ground have elements of the acrid liquid that filled the pool. The mineral build-up of the inside of this cavern leads to a direct, slick surface in the walls, and rounded off most of the stone.

Age of Arcanum
Gatshadow mountain was transformed in an evil place. As Acek Orattim, a priest of the Betrayer God the Chained Oblivion, made it his seat of power. The power of the Oblivion’s corrupted the mountain range to making this peak a jagged needle that towers over the rest of the mountains. After the banishing of the Chained Oblivion it was said that Acek’s mind and spirit were so filled with his masters essence that his body tore to asunder. Which left an empty maze of deadly chambers and traps behind beneath the mountain, forgotten and awaiting rediscovery.

Before the Stream
There was an expedition to reach the peak of Gatshadow, because there was some sort of movement, some sort of activity up there that nobody could really discern. The rumors spoke of some sort of creature that would stalk out in the night and steal mountain goats, or find and kill anybody who lived in the small village north on the mountainscape that was abandoned because of this creature the folklore of the time said supposedly lived at the top of the mountain. They found their way up to the small entrance, made their camp with a small fire but couldn't escape.

"Critical Role: Vox Machina Origins Volume 2"
Searching for their friend Grog, Vox Machina arrives at the mountain where they find an entrance. Entering the cave they encountered an abandoned subterranean city and eventually a crypt housing a mind-controlled Grog, the reanimated corpse of his father, Strongjaw, and a dying mage,Drath Mephruhn, attempting a ritual to lichdom. Under control of Drath Mephruhn, Grog, his father, and skeletons attack the party. Eventually Pike destroys Drath Mephruhn and the fight is brought to an end, bringing Grog back to himself. But not before the mage could complete the ritual and embed his phylactery into Grog's chest. After burying Stonejaw they return to Westruun, Grog with an itchy scar.

Umbrasyl made his lair in the mountain after the Chroma Conclave's attack on Emon.

After the Chroma Conclave attacked Emon, the ancient black dragon Umbrasyl roosted in a cave at Gatshadow's peak, receiving tribute from Westruun. Several scouting parties from the Herd of Storms were sent to Gatshadow in preparation for an attack on Umbrasyl, but they didn't return. Later, Shale and some other warriors from the herd scaled Gatshadow. Shale's age and disabilities slowed her down, so she survived Umbrasyl's slaughter of her party by a twist of fate, then descended from the mountain in time to meet Vox Machina.

Scaling the mountain, Vox Machina and Shale, discovered a small entrance with four long-dead skeleton corpses. There were remnants of a small camp that was long iced-over through winters past. There were roughly 85 gold pieces among their corpses and spare change in silver and copper; an oil lantern; climbing gear for four people; some long-spoiled rations; and a very moldy diary. They ascended Gatshadow further and slew Umbrasyl in his cave.