Gathering a Nightmare Vine has a chance to place a poison debuff on the gatherer called Nightmare Pollen that does 75 to 125 nature damage per second for 10 seconds.
Gathering a Nightmare Vine will also occasionally yield a Fel Lotus in addition to the herb.
Patch changes
/Patch 3.0.2 (14-Oct-2008): (Undocumented) Nightmare Seeds will no longer drop when harvesting Nightmare Vine. Patch 2.1.0 (22-May-2007): The poison from picking this herb is now considered Nature damage instead of Physical damage, and has a reduced chance to poison the herbalist.