You are playing Nightmare mode.

Setup: Each player gives 1 poison to a hero he controls.

Hundreds of angry spiders were goggling at them all round and about and above. It looked pretty hopeless. –The Hobbit

Begin with the standard quest deck and encounter deck for the Flies and Spiders scenario.

Remove the following cards, in the specified quantities, from the standard encounter deck:

3x Dark Bats

3x Hidden Path

2x Fighting Among Friends

5x Wicked Spider

1x Fat Spider

1x Giant Web

Then, shuffle the encounter cards in this Nightmare Deck into the remainder of the standard Flies and Spiders encounter deck.

Finally, flip this setup card over and place it next to the quest deck. Its effects remain active throughout the game, which is now ready to begin.