Saviour's Rocks are perhaps the original home of the eye-stealing sorrow-spiders that plague London. They are certainly where these are most numerous. The few humans living in The Nativity, the only human settlement on these islands, refuse to speak of spiders, only of "their generous neighbors of whom they will speak no ill." The humans donate eyes for the sorrow spiders to reproduce. During the Festival of Silk, humans are ...erm, sacrificed... to the spiders.
The group of islands that make up Saviour's Rocks is guarded by the Tree of Ages, a living ship of huge spider-councils, made from a complex series of deals the spiders struck and betrayed.
There are other councils besides the Tree of Ages - for example, the Tree of Seasons, which is smaller.