Spore Field

The overwhelming vigor of the fungal colonies that suffuse the Living Grotto and nearby tunnels manifests most dangerously in spore fields. These places transfix creatures that breathe in their intoxicating spores, even as other spores begin the job of rapidly decomposing the soon-to-be organic detritus.
    Spore field patches are found in areas of 3 to 12 contiguous squares. A spore field can be identified with a DC 25 Dungeoneering check.

    Effect: A creature that enters a spore field or ends its turn in one is restrained and takes ongoing 5 damage per tier (save ends both). If a creature drops to 0 hit points while in a spore field, its body produces a burst 1 centered on the creature; all squares in the burst become spore field squares. Creatures in the burst are treated as if they had entered it.
    Plants are immune to the effects of a spore field.
    Usage: Placing a spore field in an encounter with slower creatures could even up the odds a bit for those creatures. Of course, characters can also use the terrain to slow down faster foes by maneuvering the foes into it.

Published in Underdark, page(s) 109.