Nominating: Marshadow

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What effect did Marshadow have on the Metagame?

Marshadow is a really good offensive threat in the Ubers Metagame, as its high Speed and its good Attack-stat allows it to revenge kill a multitude of Pokémon in the Ubers Metagame.
This Pokémon is a powerhouse with 125 Attack and 125 Speed and has a decent bulk as well with 90 HP, 80 Defense and 90 Special Defense. Its typing is also very excellent and unique with Fighting/Ghost.
Foremost its typing in Fighting/Ghost allows it to scare out a good portion of mons in the Ubers metagame such as specific Arceus variants being Arceus-Normal, Arceus-Dark, Arceus-Ghost and Arceus-Steel.
It forces a good amount of other offensive and defensive Pokémon out: Ferrothorn, Dusk Mane Necrozma, Lunala, Dawin Wings Necrozma and Giratina-O.
Its effect left a big Impact with Arceus-Normal trending from the Max-Att and Max-Speed spread to almost Max-HP and Max-Att with 16 Speed and the Item Chople Berry to have a better time against Marshadow,
Furthermore it made that Arceus-Fairy and also Arceus-Flying have a good spot in the Ubers Metagame as they can defensively check Marshadow pretty well with their typing resisting the fighting coverage of
Marshadow pretty well and with their good bulk they even can tank a Spectral Thief and revenge it with their super effective Judgments.
Furthermore Mega-Salamence proved to be a threat to Marshadow because prior to its Mega-Evolution it has Intimidate to lower Marshadows Attack significantly which causes a switch-out at that point
because Marshadow has to fear a super effective Return / Frustration or Double-Edge on the offensive Set of Mega-Salamence and Body Slams on the defensive Sets is not only Super-effective but also has a chance to paralyze the opposing target.
Marshadow also caused a more defensive approach on the teams when it was new to the Ubers metagame and it still proves to be a threat for unprepared teams. In overall Marshadows impact was pretty high as it caused either a complete set-change (In case of Arceus-Normal)
or a change in the team archetype being a more defensive approach.

In what main roles was Marshadow used?

Its main roles are:
Life Orb All-Out-Attacker
[hide]Marshadow @ Life Orb
Ability: Technician
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Spectral Thief
- Close Combat
- Shadow Sneak
- Hidden Power Ice / Rock Tomb
The 4th Moveslot can change between HP Ice to hit Zygarde Complete and Mega-Salamence or Rock Tomb to hit Ho-Oh [/hide]

Bulk Up + Z-Move
[hide]Marshadow @ Marshadium Z
Ability: Technician
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Bulk Up
- Spectral Thief
- Close Combat
- Shadow Sneak
This set especially offers a high damage output with Bulk Up not only giving a +1 in Attack but also its Defense. The key-point here is the Z-Move (Soul-Stealing 7-Star Strike), which allows it
to Wallbreak easily and threaten everything [/hide]

Other but lesser seen Options are: Choice-Sets such as Choice Band and Choice Scarf, but both sets are easily checkable making these sets less attractive.
Sub‌itute + Drain Punch is also used but only very rarely so the chances to run into this set are really slim. Same mention goes to a Force Palm Set to have the chance to paralysis the opposing target.
Hidden Power Flying is a nice option for hitting Buzzwole but this Hidden Power typing is very rarely on Marshadow.

In overall the Life Orb and the Z-Move Sets are superior to the other Options - which also is reflected by the usage of other Sets - compared to the Life Orb or Bulk Up + Z-Move-Set.

What caused it to have a significant impact?

Its impact is caused by its signature move in Spectral Thief because this Attack is able to steal the opposing Poémons' boost such as a Swords Dance boost, Calm Mind boost, or other Bulk Up boosts
and Marshadow is capable of using these boosts to its own advantage without having the need to boost itself up with its own Bulk Up. Also on the Life Orb Set stealing a Swords Dance boost makes it even more threatening as it doesn’t only have the Life Orb
boost but also the opposing targets boost for its own advantage.
Its typing in Fighting/Ghost makes it also capable of dealing with threats which a pure Fighting or a pure Ghost-typing can't do, such as threatening an Arceus-Dark or threatening different variants of Psychic-pokémon in the Ubers tier.
Even tho there are multiple ways to deal with Marshadow it still poses a threat in the Uber Metagame as it can steal boost, using the stolen boosts to its own advantage, having a high Speed-tier and a good offensive typing in Fighting/Ghost.

How do/did you deal with this Pokémon in Ubers?

Fairy-types are a good way to deal with Marshadow the best one here is Arceus-Fairy as it can check it reliably with Recover and Judgment and it also resists its STAB in Close Combat and can - with its natural good bulk -
tank a Spectral Thief into a Shadow Sneak. Same mention goes to Xerneas as both these Fairy-types can only get OHKOed by a boosted Z-Move. Marshadow would like to have them weakened first.

Yveltal is another good check here with its well rounded typing in Dark/Flying, it can come in on both of Marshadows STABs and can revenge and soak up HP with its Oblivion Wing.

Zygarde-Complete is nearly unbypassable for Marshadow as its sky-high HP-stat and Defensive-stats allows it to tank a lot of physical moves and even boosted physical moves of Marshadow. and Hidden Power Ice
isn't a OHKO due to Zygarde-Completes superb bulk and Zygarde threatens a Glare on Marshadow and Marshadow doesn't wanna get glared.

Flying-types such as Ho-Oh and Arceus-Flying can force it out due to their STAB Options, Mega-Salamence gets a special mention due to its Ability prior to Mega and its good bulk and typing.

Faster Attackers: Choice Scarf Users mostly take little from Shadow Sneak but are capable of revenge killing Marshadow in Return.

Physically Bulky Pokémon such as Toxapex, Primal Groudon and Landorus-Therian are capable of taking on Marshadow and having a chance to scare it out either with a strong Move or a Scald Burn (In Toxapex' case).

Mega-Sableye can threaten it out with a possible Will-o-Wisp burn thus making Marshadow incapable of breaking in the later game. Faster Attackers on Full HP such as Mega-Mewtwo Y and Ultra Necrozma
can also threaten Marshadow out or beat it.