It seems that the Dahlrhun (singular and (in this case) plural) are woodlands (and the animal life within them) that have some form of comunal sentience.
They seem to chose a humanoid to speak for them and act as an intermediary with other sentient creatures. This humanoid seems to be a part of the Dahlrhun.
They are apparently very old.
They have control of some extent over nature in their realm.