Therazane is a powerful Earth elemental who has been imprisoned by its previous owner in a magical prison, which is located inside elven ruins found just underneath the Moldavie Residence. Killing it is the final requirement for the short Doors Slamming Shut Contract, given by Kurt Dysart in Novigrad.
I do not believe in ghosts - it is that simple. But my men say the house shakes at night, walls crumble, why, even the floors move about... – Kurt Dysart, Count of Anchor
There's no such thing as a risk-free real estate investment. It might turn out your new home has a leaky roof, structural rot in the attic, a scratched floor in the living room... or elven ruins beneath its foundations, and within them an enormous earth elemental just waiting for a chance to break free of its chains.
Fighting such a monster is no easy task. Its tough outer husk deflects all but the strongest blows. It can liquefy a man's bones with one crushing blow from its fists - thus, one should never near it without first casting the Quen Sign. One should have the Yrden Sign at the ready as well, whereas the other signs - Igni, Aard and Axii - are completely ineffective against it.
In the end the witcher triumphed over the powerful earth elemental, and in doing so won gratitude of, and a reward from, Count Dysart.