Spiders? Lots of spiders! Art from Sunless Sea.

Saviour's Rocks are perhaps the original home of the eye-stealing sorrow-spiders that plague London. They are certainly where they 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 Tree of Ages. Art by Paul Arendt.

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.

Spider2 The spider-councils are born from...
the eyes of large sea monsters that saw the Correspondence.
Spider What will be the spiders' biggest and final council?
The spiders plan to form the Tree of Eons. They just need to betray one more entity.