A council of sorrow

 is a Sunless Sea Story Event.

Trigger conditions
 is triggered after completing The Web of Stone for the first time, during the Story in Saviour's Rocks, when you do the Summoned by the spider-council action.

Story description
"The Emissary of Spiders stands before the Tree of Seasons, vast ranks of his masters sitting still as the rocks on its webbed branches. The subject of their interest: the legendary lost ship, the Nocturne."

Interactions
Not much happens here beyond some exposition, avoidance from the spiders about the actual goal they have for you, and progressing the status of Caught in the webs of Saviour's Rocks from "A Supplicant of Spiders" at the end. If you failed to do any spider spying in The Web of Stone, this isn't going to help explain what's going on.