Eumachia was a widow in Pompeii and was a priestess of the Triad, Jupiter, Juno and Minerva. She believed Melanie Bush and the Seventh Doctor were liars pretending to be messengers from Isis, and framed Mel for stealing some jewels to discredit the goddess.
She rescued the Doctor's TARDIS from a collapsing building, but then moved it to her family's tomb, believing it to be a temple to Isis and hoping to hide it from the people.
She thought that the volcano was the wrath of her god punishing those who believed in Isis, and forced all of her servants, including Tibernus, to remain with her as the volcano erupted. (AUDIO: The Fires of Vulcan)