A giant spider den will spawn when a shrine is looted and had fungal growth as it's description. The den will actually spawn on any tile and create a mountain even on rivers or floating ice. The dens spawn in packs of three separated from each other.They have a chance every day to spawn a Giant Spider which will head for the explorer.
The den requires a torch or pumpkin lantern to explore, similar to caves. These caves can be explored to stop the spiders from spawning. Once you enter you will start combat with 2 Giant Spiders. There is no special reward from this battle though beyond regular Giant Spider drops.