"Ori! I thought i had lost you to Kuro's talons of hate... but it was Gumo who saved you, delivering us from an uncertain fate. But look here! With the Ginso Tree restored, Nibel's waters are clear!" - Sein to Ori after escaping the Ginso Tree.
Thornfelt Swamp is a location in Ori and the Blind Forest.
At first, it is seen as the location outside of the Ginso Tree, where you first encounter Gumo, who is stealing the Water Vein. He flees, into Moon Grotto, and then to Gumo's Hideout, the lower part of Moon Grotto.
Ori is then attacked by Kuro after reaching the top of the Ginso Tree, and she pushes them off the Ginso Tree where Ori eventually lands in the Thornfelt Swamp, caught by Gumo. This is the location that Ori can learn the move Stomp. The two parts of Thornfelt Swamp appear quite separated, the only non-secret way is to go through Moon Grotto.