Probably this move called ''Nightmare''
Nightmare will reduce the target's health by one quarter of its maximum HP at the end of each turn ( Has to be asleep )
Gengar can learn a variety of moves. Gengar is a Pokemon that benefits from comboing lots of moves together, such as Hypnosis and Hex. First using Hypnosis puts the opponent to sleep, then Hex which is a 60 power move doubles in power when the opponent if inflicted with a status condition. A whopping 120 power + STAB bonus and with Gengar's excellent attack stat.
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