The Shademirk Bog, also known as the Shademirk Forest, was a corrupt, thickly-wooded swamp in the Feywild. A series of enchanted rivers, collectively known as the Gilded Run, surrounded the Shademirk Bog and prevented the corruption from spreading elsewhere in the Feywild.
The trees of the Shademirk Bog had thick trunks with sagging canopies like weeping willows. Black, sticky, tar-like ichor coated the vegetation and seeped into the ground. This viscid substance could impede foot travel; Scanlan, for example, tired from repeatedly extracting his feet from the tar.
The source of the ichor was a corrupted archfey name Saundor. After Vox Machina slew him, the black ichor that seeped out of the plants of the Shademirk Bog ceased flowing and started hardening like amber. Usin Commune with Nature, Keyleth confirmed that the surrounding plants of the bog began to relax upon Saundor's demise.
When Vox Machina returned to the Feywild about a year later, they found that the corruption had entirely receded. The area looked like a normal forest by Feywild standards.
The Shademirk Forest seemed to have some level of intelligence and recognized the people who freed it from Saundor's influence. As Vox Machina were making camp for the night, one of the trees bent its branches in a certain way to make sleeping more comfortable.