Gatshadow Mountain

, also called simply Gatshadow, is 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 of its ominous appearance, 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 is over a mile high. Gatshadow's peak is nearly always obscured by storm clouds or mist. Most of the trees are dead or dried out towards the top, but other than that the vegetation is pretty sparse.

The actual stone that comprised this monstrous formation is of a darker, more dense stone and it is 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.

Cave Entrance
Around the big cavern entrance high in the mountain, there are clumps of dead, petrified trees. Leafless, gnarled, and devoid of any life on top off melted stone and soot. The entrance is an opening on the side of the mountain face 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 Cave
This hole continued for 15 feet into the mountain before it ended abruptly. A layer of dust coats everything, and the site has remained 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 and the Calamity
Tharizdun, the Chained Oblivion had been imprisoned under Gatshadow Mountain since the Founding. A priest of Tharizdun, Acek Orattim, based himself at the mountain, which, under Tharizdun’s influence, then rose into a sharp spine much taller than the other Cliffkeep Mountains, and it was riddled with a maze of tunnels. Acek spread Tharizdun's corruption from Gatshadow to the surrounding region. During the Calamity, Tharizdun escaped his prison and went on a rampage, and when Pelor, the Dawnfather, crushed Tharizdun in battle in the Alabaster Sierras, the Betrayer God fell back to Gatshadow, but Pelor pursued and used four Prime Trammels banish Tharizdun from the Material Plane. Some say Acek was so suffused with Tharizdun's power that the god's banishment caused Acek's body to be ripped apart, leaving his base vacant to attract other villains and devotees of the Chained Oblivion.

Before the stream
Long years before Vox Machina visited the mountain, there was an expedition to investigate rumors of a nocturnal creature at Gatshadow's peak that was stealing mountain goats and had killed enough people from a small village northward to cause it to be abandoned. The members of the expedition found their way up to a small cave, made their camp with a small fire, but apparently couldn't escape, and four skeletons remained.

"Vox Machina Origins Volume 2"
Searching for their friend Grog, Vox Machina arrived at the mountain where they found 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, Stonejaw, and a dying mage, Drath Mephruhn, attempting a ritual to lichdom. Under control of Drath Mephruhn, Grog, his father, and skeletons attacked the party. Eventually Pike destroyed Drath Mephruhn and the fight was 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 returned 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. 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.