Spider Nest Mini World Change/Spoiler

Messages
World Board: A whole nest of spiders needs to be exterminated as Mamma Longlegs is on the loose.

Towncryer: Hear ye! Hear ye! Mamma Longlegs will not rest while she guards her cosy nest. Exterminate its kin in there to finally get rid of her.

Method
West of Venore some Poison Spiders run around and inside a small nest (here) you can fight Mamma Longlegs. Kill her three times to obtain the achievement Nestling. After having done so, you will not be able to enter the area any more.