This is an artistic T-shirt based on Erlkönig, Franz Schubert's best-known song, composed in 1815. The song is based on Johann Wolfgang von Goethe's famous poem, written in 1782, which is in turn based on an early Scandinavian legend. Erlkönig means either King of the Alders or King of the Elves in English. The image, painted in 1832, is by the German artist Carl Gustav Carus. For more information, there is a page about Erlkönig on Wikipedia.