Ground Swarms

Various types of vermin crawl through the tunnels of the Feydark, including masses of rats, beetles, tiny drakes, and other nuisance creatures.
    A ground swarm can occupy a single square or an area up to 3 squares on a side. It can be identified with a DC 20 Nature check.

    Effect: A ground swarm turns its squares into challenging terrain, requiring a creature that enters the swarm to make an easy Acrobatics check to avoid falling prone. Any living creature that falls prone or starts its turn prone in a ground swarm takes 1 damage per tier (or 2 per tier if bloodied).
    A ground swarm moves around during combat. Assign the swarm a position in the initiative order (initiative +10) and move it 1d4-1 squares on its turn, typically toward the nearest bloodied combatant.
    Any blast or burst that includes one or more squares occupied by a ground swarm disperses the swarm in the included squares.
    Usage: Using a ground swarm near other hazards or dangerous terrain features can make an encounter more dangerous. For example, consider having a swarm of beetles near a fast-moving stream so that if the characters can’t keep their feet, they end up having to deal with the current.

Published in Underdark, page(s) 100.