Mega Gengar is a temporary form, and after the battle Mega Gengar reverts back to being a Gengar.
Mega Gengar is a part of the Uber tier of competitive battling. Thanks to its ability Shadow Tag, Mega Gengar carves out a significant niche as a stallbreaker, wallbreaker, and revenge killer all in one Pokémon. This is due to its high 170 base Special Attack and 130 base Speed which allow it to outspeed a large part of the metagame and dish out significant damage. Having access to Taunt allows it to shut down defensive Pokémon from inflicting status or healing up with recovery moves, while Destiny Bond gives Mega Gengar the ability to take down a foe that it cannot take out. A Hex trapper set takes advantage of the status conditions spread in the tier while it is capable of spreading status itself with Will-O-Wisp or Sludge Bomb. Mega Gengar can make use of its high Speed and good defensive typing to take a different approach to trapping with a more passive set and eliminate defensive threats and Choice users with its Perish Song trapper set. Another variant of the previously mentioned Hex set is a Substitute + Hypnosis set that allows Mega Gengar to incapacitate an opposing Pokémon while having a buffed up Hex attack against a sleeping foe. Mega Gengar does suffer from having very lackluster bulk, especially in an offensive metagame like Ubers. It also has to often sacrifice itself with Destiny Bond to eliminate a threat, so its longevity is often short-lived. Despite those flaws, Mega Gengar is one of the tier's most significant Pokémon in the tier in terms of overall utility and remains an influential force in the Ubers tier.
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