The Lady of the Manor was a possible future version of Nyssa created when she was splintered in time by an exploding hyperspatial warp core engine. The Lady, with the Fifth Doctor who became the Lord of the Manor, were trapped in a time bubble/stasis field/time loop of the town of Stockbridge after Viridios awoke. They were subsequently imbued with Viridios's power and became his children.
Along with the Lord, the Lady lived in the stasis field for a period of time somewhere between 100,000 and a million years, feeding off of the unwanted memories of the villagers, who lived their entire lives every day, like a psychic vampire. The Lady of the Manor regained her memories of her existence as Nyssa when she attempted to feed on her younger self, but was killed by the Lord before she could oppose him. The Lord was later killed by the collapsing time bubble. (AUDIO: The Eternal Summer)