The eleven forest spirits are the visible sentience of the forests they reside in. As such, when their forest feels pain, they feel it as well. Though they appear ghost-like and weak, they are unkillable.
When Gaia dispersed Her essence to the forests and disappeared from the realms in the year 306 AF, nine spirits emerged to care for the forests in Her stead: Calliope in the Aureliana, Clio in the Western Ithmia, Erato in the Eastern Ithmia, Euterpe in the Northreach, Melpomene in the Black Forest, Polyhymnia in the forests of Ulangi, Terpsichore in the Aalen, Thalia in the Northern Ithmia and Urania in the Darkenwood.
Two more spirits have joined the nine sisters since then: Eupheme, spirit of the Aerinewild, and Propasia, whose forest was lost when Cyrene was built.