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As long as the wielder is on the field against a sleeping opponent, that opponent will take damage equal to 1/8 of its maximum HP at the end of each turn. This ability will deal damage even if the opponent is asleep due to Rest. In a multi battle, a sleeping Pokemon will lose health due to each adjacent foe's Bad Dreams. A sleeping Pokemon will not lose health because of its partner's Bad Dreams.
Darkrai may be the only Pokemon to obtain this ability, but even something as frail as Darkrai can put it to good use. Darkrai is well known for being extremely fast, and so very capable of putting a threat on the opponent's team to sleep. This will result in at least one turn of Bad Dreams damage. However, it is more likely than not that a sleeping Pokemon will switch out of Darkrai, not only out of fear of Bad Dreams, but out of fear of either a powerful switch-in, or Darkrai's powerful special attacks. All in all, this ability is decent for doing small damage, making KOs easier, but Darkrai usually will not be trapping, then stalling foes to death with it.
Found in Dream World