Druidic

Druidic is the secret language of druids, which is learned upon taking a level of druid; this knowledge is imparted by the forces of nature itself. Druids are forbidden to teach this language to non-druids, and non-druids struggle to comprehend it. It utilizes its own alphabet, which is also kept secret.
To the untrained eye, Druidic script appears to be a series of interwoven shapes, swirls, lines, and symbols carved into trees as warnings and messages to traveling druids. To the untrained ear, it blends with the wind; druids have mastered using a series of whistles, bird calls, and animal sounds to communicate with one another from both short and long distances.
Druidic cannot be learned without taking a druid level, nor can it be understood or imitated by mortal magic.