A "recreational program with light RPG elements" that posesses a very distinct visual and musical style.
This game's art is probably what it is most well-known for. It's strange and otherworldly-feeling, but at the same time you can really feel the talent and dedication behind it.
The music is another subject entirely. It's clear that a lot of the music in both Hylics and its sequel, Hylics 2, was heavily influenced by that of neo-psychedelia and prog-rock (though I think the first Hylics leans more heavily on the psychadelics, with all of its relaxed and ambient tracks). This is only supported by the fact that Hylics 2 has not one, but two prog-rock album companion pieces that you can listen to.