It can be found by going to the lower-left corner of the world map. There Ori can find Tokk, who is musing on how to get into the area.
To remove the roots blocking the way, bash off of the bell-flowers hanging from the ceiling in the order suggested by the monoliths in the background. Doing so causes the roots to pull back from the entrance. After exploring the area below, which is full of portals reminiscent of the Ginso Tree from Ori and the Blind Forest, Ori will acquire the translation rune that Tokk was looking for. Returning it to him causes him to complain that the text is backward. This is a hint to bash off of the bell-flowers in reverse order to gain access to the Ancestral Tree to the right of the area's entrance, which grants Ori Ancestral Light.