Aarni has been described as an avant-garde metal band from Finland, which consists primarily of Master Warjomaa and, occasionally, some session musicians. Although many sources claim the existence of other band members, these may be fictional characters (probably created by Warjomaa himself) and include a French count born in the 17th century (Count of Saint-Germain), a cartoon character with a T-shirt with Aarni's logo (Doomintroll) and an old woman (Mistress Palm).
Aarni's music bears similarities to funeral doom metal (in Reaching Azathoth, for example) and has sometimes similarities to folk metal (The Weird of Vipunen). Their style has been described as almost orthodox doom metallish Lovecraftian-Jungian Kalevala avantgarde music. The band themselves sometimes use the term Chthonic Musick. The lyrics of Aarni include varied themes such as Finnish folklore, transhumanism, Lovecraft, paganism, parapsychology, psychoanalytical theories and mythology. The lyrics have been sung in English, Finnish, Latin, and occasionally in Enochian, Ancient Egyptian, Ouranian Barbaric, Swedish, and Glossolalic.