Geological / environmental event
Over the course of several months of expeditions into its depths, the Darkwood Forest is progressively secured by the Heroes of Last Rest, first after successfully negotiating an alliance with the Druid's Grove, then developing relations with the inhabitants of the Necromancer's Tower, finally contracting safe pasage with the spider clans.
References:
9.27 - Spiders in the Darkwood
11.5 - Corruption in the Darkwood
11.13 - The Standing Stone
11.20 - Necromancer's Tower
12.2 - Arachnid Negotiations