Molora, Barbican Pit, London

Both chorus and conscience, the Ngqoko Cultural Group provide an extraordinary ambience and commentary in Yael Farber's South African version of the Oresteia. The six women and one man produce, from a drum, a Jew's harp, and a calabash and bow, a range of eerie thumps and twangs. They sway, stomp, brandish poles, and clap their hands, but, even at their most violent, their demeanour is always grave. Most impressive of all, these Xhosa people, from rural Transkei, produce, with their split-tone singing, a range of sounds that are human and animal, earthly and divine. They ululate and whinny, they weave complicated harmonies. They become, at one point, a herd of doom-saying beasts; at another, a flock of vengeful jungle birds.

The play itself, however, which Farber has written and directed, is not so consistently powerful. Farber concentrates on the end of the story, when Klytemnestra receives a visitor who she does not know is her son, Orestes, spirited away 17 years before by his sister, Elektra. Before this, we hear Klytemnestra recount the murder of their father, Agamemnon, and see her trying to force Elektra to reveal where she has taken Orestes. She does this with the means of torture and intimidation used on dissenters in the days of apartheid: the plastic bag tied over the head, the burning cigarette, the whip.

For the Greek revenge play is here tied to the recent history of South Africa, where rulers and the conquered have had to learn to be equals. One aspect of this process was the Truth and Reconciliation Commission, which, from 1995 to 1998, heard testimony from both oppressors and victims. Klytemnestra and Elektra allude to this by sitting behind a wooden table and speaking into a microphone. Elektra has been punished for her hatred of her mother, during Orestes' exile, by being forced to wait on table and eat on the floor; in the last lines of the play the white Klymtemnestra (Elektra and Orestes are black) delivers her judgment on white South Africa: "We who made the sons and daughters of this land servants in the halls of their forefathers, we know we are only here by grace."

The line is a magnificent one, but elsewhere Farber can sound portentous and opportunistic. Klytemnestra gleefully recalls hacking her husband to pieces: "The murderous shower wounds me, dyes me black. I revel like the earth when the spring rain comes down."

Elektra, quoting Shylock's speech justifying revenge, merely disconcerts us. The acting is uneven too. Jabulile Tshabalala (Elektra) and Sandile Matsheni (Orestes) convey their anguish with a moving delicacy and restraint, Mathseni especially touching in the contrast between his hesitant, fearful approach to matricide and his sister's furious demand for blood. Dorothy Ann Gould's Klytemnestra, however, her mouth fixed in a snarl, her voice throbbing with monotonous melodrama, arouses only irritation. Standing atop a table, holding an axe over her head with both hands and shrieking, she could be demonstrating that more is less.

The visual aspects of Molora are more inventive and disturbing. At the centre of the stage, as it is of the drama, lies a mound of earth, Agamemnon's grave. A table is placed over it on which Klytemnestra dines with the son who has come to turn her into meat. Orestes raises the axe over her but, at the height of his swing, freezes, and Elektra snatches it from him, ready to bring it down. In a heart-stopping moment, the chorus, stepping from the sidelines into the drama, surround her, disarm her, and carry her away, murmuring and caressing. "Molora" is the Sesotho word for "ash," and, as we are reminded, when fire is met with fire, all that will remain is ashes.

To 19 April. (020-7638 8891)