Literal translation -
"By command of the High Priestess
We need more slaves! Mine of black rock is falling."
Not completely clear, and I think there's a slight confusion of words - Usst usually means 'mine' in the sense of 'belonging to me', not 'hole in the ground where you dig up stuff'....
(I'm not too certain about using d' to mean 'of' in a general sense, I think it usually means 'of' in the sense of 'belonging to a family, city, country', but I'd need to do some hunting for the books I used to use for this stuff!)
I'm inclined to think it means 'we need more slaves because the production rate of blackrock in the mine is falling', but I suppose it could also mean 'the mine is collapsing'!