Shadowfell

The Shadowfell is one of the Planes of Existence in the Critical Role universe. The Raven Queen's icy realm of Letherna is tucked into a corner of the Shadowfell. The plane was also the home of Thar Amphala until that city was teleported by Vecna to the Material Plane.

Description
According to Eskil Ryndarien, the Shadowfell is a plane of negative energy, and elements of it are used to funnel the transition of souls from life to death or back. It exists as a mirror plane, much like the Feywild. There are elements that resemble Exandria, but twisted, shifted."

When Vex'ahlia shot an Oracle Arrow into the siphon beneath Whitestone, she caught a glimpse of the Shadowfell on the other side:

History
Ages ago, the silver dragon Karkethzerethzerus (Karketh for short) used the energies of the Shadowfell to annihilate a nation of wood elves and centaurs, turning that region of the Miskath Strand into a petrified woodland now called Strathfell. As of 836 PD, Karketh is a shadow dragon, and his evil fey minions like darklings and meenlocks travel through rifts between Strathfell and the Shadowfell. They do so through via the Border Ethereal.

In the original Tal'Dorei Campaign Setting, the benevolent fey of the Frostweald are using the forest as a refuge from a "Great Shadow" that has befallen their lands in the Feywild, possibly referring to an army from the Shadowfell.

In the Explorer's Guide to Wildemount, the adventure Unwelcome Spirits involves a shadow demon named Trush attempting a ritual from the Shadowfell, manipulating the veil between the Shadowfell and the Border Ethereal to possess a warlock. In the adventure hook Best Left Alone, an unearthed relic of the Shadowfell begins to summon gloomstalkers and shadowghasts into Xarzith Kitril.

Vecna and Thar Amphala
Ioun told this history to Vox Machina in . Shaun Gilmore believed these events happened much later.

A century before the Calamity, the archmage Vecna achieved lichdom, amassed followers and undead forces, and with them disappeared into the Shadowfell, where they conquered the city of Thar Amphala and there built a tower called Entropis that served as his base. From there Vecna exploited the celestial solstice, a merging of ley energies, that gave him the power to open portals at a whim, so that his forces could strike with precision and retreat to Thar Amphala before retaliating forces could muster. Thus he overcame some old enemies and forced them to bend the knee, at which time he slew them and raised them as undead to serve him. One such rival, Kas, was offered vampirism in exchange for fealty as a Vecna's chief lieutenant, and Kas accepted.

Vecna forged for Kas a relic blade with a fraction of Kas's consciousness. In the months of terrorizing Vecna's enemies that followed, he became Kas the Bloody-Handed.

Then, Vecna's foes figured out how to reverse-engineer the celestial solstice. Just as Vecna was attempting to ascend to godhood with the Ritual of Seeding atop Entropis, a holy army led by Yos Varda attacked Thar Amphala. Yos Varda climbed Entropis and fought Vecna directly. Vecna defeated Yos Varda but was winded by the effort, and as Varda lay dying, Kas attacked Vecna with that gifted blade in an attempt to usurp the lich. The ensuing battle destroyed them both: all that remained of Vecna were his left eye and left hand, and all that remained of Kas was in that blade. Very few members of the holy army returned from the Shadowfell.

Centuries later, starting at least as early as 810 PD, Vecna's followers began setting up siphons at ziggurats to facilitate the collection of magic artifacts and ley energy to Entropis, gathering power that would be used in the ritual to resurrect the lich.

In, in 812 PD, Vox Machina chased a resurrected Delilah Briarwood into the Shadowfell, following their encounter with her in a ziggurat in Marquet. There they witnessed, from a distance, Vecna's rebirth atop Entropis. They fought a losing battle against Vecna and his lieutenants and had to flee into the Feywild. Shortly thereafter, Thar Amphala teleported out of the Shadowfell to Exandria.