Order and DestructionEdit
Kill Magma Crawler Giant Spiders around 43000, 7500 in Thunder Mountain until you gather 5 Arachnid Poison Sac from them. Right-click the stack of Arachnid Poison Sacs in your inventory to obtain the unlock. The drop rate on Arachnid Poison Sac is around 3%, so expect to need to kill 100-200 spiders for the unlock on average.
Also dropped from Terrorfang Spiders in Caverns of Terror, north Order side Chaos Wastes.