Umbra Hills

The  are a blasted wasteland at the base of the Cliffkeep Mountains, cursed forever by a tide of spilled demonic blood across the highlands.

Description
No animals live here, and the only plant that grows in these hills is shadegrass. The grass's unique acrid flavor has drawn the interest of spice traders worldwide, and they hire armed escorts for their spice-pickers.

Undead soldiers rise from the grass when the moon is high and the centuries-old ruins of Drassig war camps draw shadow demons to patrol these lands, still bound by a contract between Trist Drassig and Graz'zt, the Demon Prince of Indulgence.

The Tal'Dorei Council's influence rarely touches Jorenn Village to the south, much less the Umbra Hills themselves.

Battle of the Umbra Hills
As the Scattered War grew increasingly desperate for King Trist Drassig, Zan Tal'Dorei's rebel human forces and the allied Elves of Syngorn backed Drassig's army into a valley in the Umbra Hills. Obsessed with the possibility of losing, Drassig pledged his service to Graz'zt, for whom the nearby Grey Valley had once been a bastion of influence, in exchange for the power to smite his foes. The bargain accepted, during the battle a flood of fiends, including Graz'zt himself, poured into the battlefield in support of Drassig. All the continent's greatest mages fought in the battle. The hills ran dark with human blood and demonic ichor. Against all odds, Zan Tal'Dorei's forces prevailed, ended the Drassig bloodline, and banished Graz'zt back to his Abyssal prison.

However, the demonic blood that spilled that day permanently transformed the hills from a floral, sun-dappled emerald highlands into a blasted wasteland; grass and flowers grow burnt and black, their sap coursing with demon blood.

Silver Rush
Around 745 PD, more than two centuries after the Battle of the Umbra Hills, rich silver deposits were discovered in nearby hills, drawing prospectors and common folk to Jorenn Village despite the nearness of the still-demon-haunted Umbra Hills. By 802 PD, a small group of mercenaries and vigilantes formed the Shadewatch to hunt the creeping dangers of the night.

Campaign One: Vox Machina
The prison in which Vox Machina first found Percival de Rolo was located in the Umbra Hills far north of Westruun.

By 812 PD, Shademaster Arhanna Lewyn was sending a trio of Shadewatch agents into the Umbra Hills or to Gatshadow Mountain weekly.

One adventure hook offered in the Tal'Dorei Campaign Setting sourcebook, set in 812 PD, involves the discovery of a magic longsword inscribed with the words "Trist Drassig" in the Umbra Hills, but those who claim the blade are, starting one week later, attacked by shadows nightly.