Millions of years in the past, an advanced civilisation on Betrushia created an organic catalyst. It was meant to test species for survival traits, but it worked too well and consumed any living thing. Before being wiped out, the inhabitants built an artificial ring system to confine the catalyser to Betrushia.
By 2148, the new inhabitants of Betrushia, the Cutch and the Ismetch, were at war with each other. The Seventh Doctor and Benny became involved in the war, but it became moot when the Chapter of Saint Anthony targeted the planet, inadvertently freeing the catalyser. Betrushia was destroyed. (PROSE: St Anthony's Fire)