Dust of a Thousand Spiders
This dust when thrown into the air turns into a swarm of normal poisonous spiders.  The swarm acts just a summon swarm spell by 3rd level caster.  It cannot be directed but will follow the last creature to leave its area with a speed of 30 feet.  As the individual members of the swarm die they turn into (non-magical) dust.  The swarm as a whole disintegrates after 5 rounds.
    
Caster Level: 3rd; Prerequisites: Craft Wondrous Item, summon swarm; Market Cost: 300 gp.
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