Diamanda Galas, the diva of the avant-garde whose amazing voice spans three and a half octaves, is back in town with her latest one-woman vocal extravaganza. Masque of the Red Death, which she calls a "plague mass," is an operatic triptych dealing with the AIDS crisis. In the first part snippets from Leviticus, Job, and the Psalms are used to set up an ironic exchange between a cruel God and a quarantined man. in which, she says, "harsh pronouncements are juxtaposed with lamentations from a hunted criminal." The centerpiece is a dirge of mourning in which Galas sings of vengeance for the dead, and in the last installment, done gospel style, she takes a knife in hand and challenges the devil from a pulpit. The hour-and-a-half mass, with texts in five languages, and accompaniment from percussions and a synthesizer, plays like a series of primal screams alternating with interludes of Berliozian grandeur. Its ferocious energy has earned it comparison to an earlier audience shocker--Stravinsky's The Rite of Spring--and its morbid, rebellious sentiments are drawn from the tradition of Poe, Sade, and Baudelaire. Galas says her "charting of the geography of a plague mentality" is a work in progress (begun in 1984) for as long as the epidemic lingers and prejudices prevail. After her performance, healthcare experts invited by the Howard Brown Memorial Clinic will be on hand to conduct a forum on AIDS and on the emotional issues raised by the mass. Wednesday, 8 PM, the Vic, 3145 N. Sheffield; 242-6237, 663-1628, or 472-0449.