Bonnie 'Prince' Billy, Royal Festival Hall, London

In the memorable 2005 song "Williamsburg Will Oldham Horror" by the anti-folk singer Jeffrey Lewis, Oldham, who also goes by the stage name Bonnie "Prince" Billy, is referred to as the "king of a certain genre". Among the loyal subjects at a packed Royal Festival Hall, adoration is high on the agenda. Throughout the show, the crowd sits in quiet reverence, hanging on the performer's every word, waiting patiently while the band tunes up. No chatter can be heard and the silence is only broken by large cheers at the end of each song.

This reverence and adoration reaches its height during "I See a Darkness", perhaps his best-known song, which Johnny Cash covered in 2000. Slow and stately on record – from the album of the same name – tonight Bonnie "Prince" Billy and his band strip the song back even further, creating a thing of spare beauty. The audience is entranced.

What that "certain genre" amounts to is Oldham's very own kingdom of Americana, which he has been mapping under various guises since the mid 90s. It encompasses all the alternative labels you can think of – alt-country, alt-rock, alt-folk – but also embraces more mainstream conceptions of country and folk as well as bluegrass. Throw in a visual aesthetic resembling a character from Deliverance, a deadpan and surreal sense of humour and some gothic lyrics and you're close to the genus that Oldham has created.

Also key to this kingdom, it seems, are some serious beards, which are on display both on stage and among followers. Fleet Foxes and Bon Iver may have created a vogue for wild beards last year, but Bonnie "Prince" Billy and his merry men got there long before.

What is most impressive about Bonnie "Prince" Billy live is his ability to reinvent his songs and his whole show. This is the fourth time I've seen him play and each gig has been entirely different: from a simple one-man-and-his-guitar show nine years ago to a fully fledged band on this occasion – with various other incarnations in between. The latest version sees him backed by players on double bass, guitar, violin and drums, with Oldham on electric guitar. Particularly noteworthy is the Australian Jim White, of the Dirty Three, whose smooth drumming is impeccable throughout.

The show works best during the fast, soaring numbers and the quieter songs. "Beware Your Only Friend", from the latest album, Beware, is a standout. With powerful guitars it sounds huge, enveloping the grand stage, only to break down into a beautifully sparse middle section before soaring again. On other, quieter efforts, the band sounds fascinating, creating strange soundscapes that veer into experimental territory. On some of the mid-tempo tracks, however, things blur slightly into one another, and with the fiddle at the forefront, one or two of these songs move just a little too close to country and western standards for comfort.