Gatshadow Mountain

Gatshadow Mountain, also called simply Gatshadow, is an enormous mountain in Tal'Dorei at the southeastern tip of the Cliffkeep Mountains, just northwest of the Bramblewood Forest and Westruun.

General description
Gatshadow's name is a nod to its ominous appearance. Over a mile high, it towers over the smaller mountains that surround it, and is made of darker stone. The slopes of the mountain have sparse vegetation, with very little at the peak.

The dark stone of Gatshadow is particularly dense. It is considered to be the finest stone for crafting on the Tal'Dorei, and artisans often seek it out, even going to the mountain itself, despite the dangers associated with the climb.

Cave entrance
The main entrance to Umbrasyl's lair is surrounded by dead, petrified trees, and is surrounded by soot, which Vox Machina surmised may have been due to a long-ago fire. It cannot be reached by foot, as there is no path to it, just the cavern in the side of the mountain. There is a smaller entrance about 60 feet below it, but it does not connect to the main cavern and appears to stop about 15 feet into the mountain. When Vax explored it, he found the corpses of several mountain climbers and a mostly ruined diary concerning an expedition to the peak to investigate strange activity.

There are two huge nearly-identical rocks embedded in each side of the cavern wall at the main entrance. They appear less weathered than the surrounding stone, and are part of an enchantment.

Cavern
The cavern is full of pools of acid and multiple stone outcroppings, including a large platform-like slab The ceiling is 70 feet high, and there are stalactites and stalagmites throughout the cave, some of which meet as pillars.

The minerals and acid within the cavern had eroded the stone so that the floors and walls were slick and smooth.

Pre-Divergence
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 died in the small cave below the larger cavern that would one day become Umbrasyl's lair.

"Vox Machina Origins Volume 2"
While searching for their friend Grog, Vox Machina arrived at the mountain where they found an entrance. Upon entering the cave, they encountered an abandoned subterranean city, and eventually reached a crypt which housed a mind-controlled Grog, the reanimated corpse of his father, Stonejaw, and a dying mage, Drath Mephruhn. Drath Mephruhn was attempting a ritual to lichdom. Under the 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, Vox Machina 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, and then descended from the mountain in time to meet Vox Machina.

As Vox Machina and Shale climbed Gatshadow, Vax found the remains of the expedition to the peak and read the moldy diary they had left. The party ascended Gatshadow further and entered the cavern, where Umbrasyl lay invisible. They fought the dragon, and Grog landed the killing blow.