The Katachthon Mountains

While folk wisdom holds that the eastern Oraniad Mountains are blessed by the gods, dark, older magic supposedly holds sway in the Katachthons. While the Katachthons are a smaller range than the Oraniads, they are harsher and more volcanically active, with regular avalanches, storms, and unpredictable microclimates. Clouds obscure heavenly signs, guides become disoriented, and explorers wander in ever-smaller circles. And above it all, dragons rule the range—fire-belching creatures whose bellies, it is said, are filled with the flames of Purphoros’s forge.
Those who live in the foothills of the Katachthons are loath to acknowledge any tales of wondrous treasure or mysterious creatures to be found there, only repeating a warning to travelers: if you must cross them, keep to the passes. The two passes, One-Eyed Pass and the Cypress Gates, aren’t safe, but they are generally considered less lethal than attempts to blaze one’s own trail.

Mount Velus

Visible from Akros, Mount Velus is an energetically active volcano. The fiery mountain serves as nothing less than Purphoros’s home in the mortal world.
While the volcano has only erupted a few times since Akros’s founding, those minor catastrophes were enough to demonstrate that a major eruption might destroy the entire region. Akroans regularly leave offerings at the foot of the volcano, and rigorously dissuade travelers from venturing up to the sacred peak, hoping to avert the great explosion they fear is coming.
While some adventurers have climbed to the volcano’s caldera, met Purphoros, and lived, the god’s anger isn’t the only danger. A red dragon called Thraxes makes his home in the halls of Purphoros’s first residence, which the god abandoned to create a larger and more ornate home deeper in the volcano. The dragon occasionally uses the old forge to craft his own creations, melting gold from his treasure hoard. He sometimes gives these creations to Purphoros as offerings. In exchange the god allows the dragon to stay in the volcano.
Purphoros’s forge lies in the deepest part of Mount Velus, near a vast lava pool. The massive iron forge is surrounded by the finest ores, metals, and gems. Some of the greatest treasures Purphoros ever crafted are contained in the god’s residence, a forge-palace warded by an enormous, gilded iron door. Occasionally these doors crack open, admitting a curious anvilwrought creature into the world, or burst wide, unleashing some manifestation of Purphoros’s fury.

Mount Vesios

Mount Vesios, also known as the Little Sister, is a smaller volcano that resembles Mount Velus and erupts concurrently with Velus. A monastery on its western slope houses scholars and spellcasters who study the volcano, believing that through it, they might be able to control the eruptions of Mount Velus. Numerous oreads and Akroan flamespeakers offer their omens from the volcano’s heights.

Inhabitants

The Katachthon Mountains and surrounding area are populated by few mortals due to the risk posed by Mount Velus and Mount Vesios. 

At the mountains highest peaks dwell ancient dragons with little reason to venture far from their clifftop homes. 
Further down live small communities of wild hunter harpies and roaming bands of Katachthon goliaths. Oreads are a common site in this region, usually dwelling within volcanos alongside Purphoros and his creations.