Alasdair Roberts, Purcell Room, London

In a world where musicians rush to lay bare the secrets of their songs on blogs, and continually mine the same historical sources – the Sixties, the Eighties – for inspiration, the Scottish folk singer Alasdair Roberts is an anomaly. His appropriation of antique songs calls to mind the prospecting of Daniel Day-Lewis's oilman in There Will Be Blood: he performs the subterranean, hardscrabble toil of research, then plunges his drill down deep, coming back with black gold.

And it is black. Roberts has confessed to being so "troubled and depressed" by these ancient death ballads that he has to record them so that he can get on with writing his own material. That his own songs are as dark and deeply odd as the Elizabethan ballads makes you worry for his mental health.

Roberts himself is everything you might expect from a singer who cites Roland Barthes's essay "Death of the Author" in relation to his performing style. Dressed in red shirt and blue jeans, he's self-effacing and boyishly artless. "I'm just going to go and locate my cable," he mutters after walking on stage and finding that he can't plug in his acoustic guitar. A fruitless rummage behind a curtain later, he gives up and sings his opening song, the murder ballad "On the Banks of Red Roses", a cappella.

The traditional arrangements he goes on to play bear out Bob Dylan's admission that "I've never written anything as far-out as some of the old songs". Infanticide and martyred cabin boys roll past tonight like characters in some medieval David Lynch nightmare, climaxing with a horrifying version of "Long Lankin", about a stonemason who murders a lord's wife and baby. The lady offers up her other child in return for her life, but Lankin slays her anyway. It leaves you saucer-eyed with fear.

Roberts's own compositions are remarkable in their melodic sophistication and use of language: "I will go to bed/ And dream of where the stags lock antlers and give joyous bellow." Roberts fashions his songs from the stout timber of myth, and creates songs that, for all their haunting morbidity, throb with electricity. If contemporary pop sometimes feels like a Frankenstein's laboratory, where the corpse of, say, Joy Division, is constantly being jerked to brief, unwonted life, Roberts is creating a beast that's built to last.