I'm going to be voting
NO BAN. Marshadow is definitely an amazing mon, there's absolutely no doubt about that. However, I don't think Marshadow is broken by any means, and I think it offers something to the tier few other mons can.
Sablenite Blissey is a very annoying mon to face, using Toxic stall with Seismic toss to take down several top tier threats. It is only really beat by set up mons, which are easy to play around with faster things for Tail Glow Xurk, and Golisopod for double dance Groudon. Even Nasty Plot Psyshock Azelf has some trouble beating it.
+2 252 SpA Azelf Psyshock vs. 248 HP / 252+ Def Blissey: 336-396 (47.1 - 55.5%) -- 74.2% chance to 2HKO
This is coming from Diancite Azelf, which I haven't seen on the ladder yet, not to mention Azelf is by no means a common mon. However, Marshadow offers us a reliable way to beat Blissey. Being ghost type, it is immune to Seismic Toss, something no other fighting type(other than Lopunnite Dhelmise or something of that nature, which let's be honest no one is running) can say they have. When used in combination with a steel type like Magearna, which can Volt Switch out on the Seismic Toss into Marshadow and get it a free Bulk Up or Close Combat, allowing it to sweep late game.
Metagrossite Kartana also loses a check, which doesn't have too much of an impact, but Matagrossite Kartana is very fast and a huge threat, with Marshadow being able to outspeed it and easily OHKO.
252 Atk Marshadow Close Combat vs. 0 HP / 0 Def Kartana: 284-336 (109.6 - 129.7%) -- guaranteed OHKO
However, Marshadow has several common counters in the tier. There are some common tier threats that beat it, as well as some more niche sets I came up with myself.
So obviously, we'll start with the top tier threats. Zapdos, Mandibuzz, and ofc Arceus Fairy are all able to beat Marshadow 1v1. They all have access to recovery, meaning they can just Roost/Recover the hits from Marshadow off.
Sablenite Mandibuzz vs. Metagrossite Marshadow
252+ Atk Tough Claws Marshadow Close Combat vs. 248 HP / 252+ Def Mandibuzz: 153-180 (36.1 - 42.5%) -- guaranteed 3HKO
0- Atk Mandibuzz Foul Play vs. 0 HP / 0 Def Marshadow: 175-207 (54.5 - 64.4%) -- guaranteed 2HKO
0- Atk Mandibuzz Foul Play vs. -1 0 HP / 0 Def Marshadow: 262-310 (81.6 - 96.5%) -- guaranteed 2HKO(-1 because of CC)
Sablenite Mandibuzz vs. Lucarionite Marshadow
252+ Atk Adaptability Marshadow Close Combat vs. 252 HP / 252+ Def Mandibuzz: 178-210 (41.9 - 49.5%) -- guaranteed 3HKO
0- Atk Mandibuzz Foul Play vs. 0 HP / 0 Def Marshadow: 240-283 (74.7 - 88.1%) -- guaranteed 2HKO
(You get the idea for the +1 and -1 calcs for CC/Foul Play)
Let's look at Zapdos.
Venusaurite Zapdos vs. Metagrossite Marshadow
252+ Atk Tough Claws Marshadow Spectral Thief vs. 248 HP / 252+ Def Zapdos: 133-157 (34.7 - 40.9%) -- guaranteed 3HKO
0 SpA Zapdos Thunderbolt vs. 0 HP / 0 SpD Marshadow: 126-148 (39.2 - 46.1%) -- guaranteed 3HKO
Venusaurite Zapdos vs. Lucarionite Marshadow
252+ Atk Adaptability Marshadow Spectral Thief vs. 248 HP / 252+ Def Zapdos: 156-184 (40.7 - 48%) -- guaranteed 3HKO
0 SpA Zapdos Thunderbolt vs. 0 HP / 0 SpD Marshadow: 148-175 (46.1 - 54.5%) -- 56.3% chance to 2HKO
And of course, Arceus Fairy.
Arceus Fairy vs. Metagrossite Marshadow
252+ Atk Tough Claws Marshadow Spectral Thief vs. 252 HP / 252+ Def Arceus-Fairy: 136-162 (30.6 - 36.4%) -- 58.2% chance to 3HKO
252+ Atk Tough Claws Marshadow Poison Jab vs. 252 HP / 252+ Def Arceus-Fairy: 164-194 (36.9 - 43.6%) -- guaranteed 3HKO
0 SpA Pixie Plate Arceus-Fairy Judgment vs. 0 HP / 0 SpD Marshadow: 278-330 (86.6 - 102.8%) -- 12.5% chance to OHKO
Arceus Fairy vs. Lucarionite Marshadow
252+ Atk Adaptability Marshadow Spectral Thief vs. 252 HP / 252+ Def Arceus-Fairy: 160-190 (36 - 42.7%) -- guaranteed 3HKO
252+ Atk Marshadow Poison Jab vs. 252 HP / 252+ Def Arceus-Fairy: 144-170 (32.4 - 38.2%) -- 97.2% chance to 3HKO
0 SpA Pixie Plate Arceus-Fairy Judgment vs. 0 HP / 0 SpD Marshadow: 330-390 (102.8 - 121.4%) -- guaranteed OHKO
You also need to keep in mind all of these cal are done from Adamant Marshadow. No one runs that. Everyone is running Jolly to outspeed other Marshadows, Kartanas, etc. So you can see this is doing less to these mons, meaning they can roost/recover up on it and kill it 1v1 even easier.
Magearna and Pdon can check Marshadow and OHKO it without being able to be OHKOd in return.
252+ Atk Tough Claws Marshadow Close Combat vs. 248 HP / 252+ Def Magearna: 153-180 (42.1 - 49.5%) -- guaranteed 3HKO(Metagrossite)
0 SpA Magearna Fleur Cannon vs. 0 HP / 0 SpD Marshadow: 372-440 (115.8 - 137%) -- guaranteed OHKO(Metagrossite)
252+ Atk Adaptability Marshadow Close Combat vs. 248 HP / 252+ Def Magearna: 178-210 (49 - 57.8%) -- 96.1% chance to 2HKO(Lucarionite)
0 SpA Magearna Fleur Cannon vs. 0 HP / 0 SpD Marshadow: 440-518 (137 - 161.3%) -- guaranteed OHKO(Lucarionite)
252+ Atk Tough Claws Marshadow Close Combat vs. 248 HP / 0 Def Groudon-Primal: 192-226 (47.6 - 56%) -- 82.4% chance to 2HKO(Metagrossite)
252+ Atk Groudon-Primal Fire Punch vs. 0 HP / 0 Def Marshadow in Harsh Sunshine: 259-306 (80.6 - 95.3%) -- guaranteed 2HKO(Metagrossite, needs some chip)
252+ Atk Adaptability Marshadow Close Combat vs. 248 HP / 0 Def Groudon-Primal: 224-264 (55.5 - 65.5%) -- guaranteed 2HKO(Lucarionite)
252+ Atk Groudon-Primal Fire Punch vs. 0 HP / 0 Def Marshadow in Harsh Sunshine: 262-310 (81.6 - 96.5%) -- guaranteed 2HKO(Lucarionite, needs some chip)
On top of these common top tier counters to Marshadow, I have some of my own to talk about that aren't used right now but can still beat it 1v1.
These two mons get Wish, and are super effective vs. Marshadow. However, their defense is lackluster. That's where Slowbronite comes into play.
Slowbronite Florges vs. Metagrossite Marshadow
252+ Atk Tough Claws Marshadow Poison Jab vs. 248 HP / 252+ Def Florges: 146-174 (40.6 - 48.4%) -- guaranteed 3HKO
252+ Atk Tough Claws Marshadow Spectral Thief vs. 248 HP / 252+ Def Florges: 124-147 (34.5 - 40.9%) -- guaranteed 3HKO
0 SpA Florges Moonblast vs. 0 HP / 0 SpD Marshadow: 254-302 (79.1 - 94%) -- guaranteed 2HKO
Slowbronite Florges vs. Lucarionite Marshadow
252+ Atk Adaptability Marshadow Spectral Thief vs. 248 HP / 252+ Def Florges: 146-172 (40.6 - 47.9%) -- guaranteed 3HKO
252+ Atk Marshadow Poison Jab vs. 248 HP / 252+ Def Florges: 130-154 (36.2 - 42.8%) -- guaranteed 3HKO
0 SpA Florges Moonblast vs. 0 HP / 0 SpD Marshadow: 306-360 (95.3 - 112.1%) -- 68.8% chance to OHKO
Marshadow does even less damage to Sylveon and Aromatisse due to them having a higher HP and Defense stat. They're damage output is:
0 SpA Sylveon Moonblast vs. 0 HP / 0 SpD Marshadow: 254-300 (79.1 - 93.4%) -- guaranteed 2HKO(Metagrossite)
0 SpA Sylveon Moonblast vs. 0 HP / 0 SpD Marshadow: 300-354 (93.4 - 110.2%) -- 56.3% chance to OHKO(Lucarionite)
0 SpA Aromatisse Moonblast vs. 0 HP / 0 SpD Marshadow: 236-278 (73.5 - 86.6%) -- guaranteed 2HKO(Metagrossite)
0 SpA Aromatisse Moonblast vs. 0 HP / 0 SpD Marshadow: 278-330 (86.6 - 102.8%) -- 12.5% chance to OHKO
Oh, what the hell am I talking about? All 3 of these are weak to Close Combat. Well my friends, not with the Audinite Mega Stone! Lickilicky and Umbreon both get Wish(as well as Umbreon getting Moonlight) so they have recovery, while Heatran doesn't but I've seen it before. These are not the only Pokemon that when given the Audinite can beat Marshadow. Lots of mons can, pretty much anything bulky that has recovery and isn't weak to Ghost. I can see some mons working with the Altarianite because of Pixilate, but Audinite gives more bulk.
252+ Atk Tough Claws Marshadow Close Combat vs. 248 HP / 252+ Def Lickilicky: 169-199 (39.9 - 47%) -- guaranteed 3HKO(Metagrossite)
252+ Atk Tough Claws Marshadow Poison Jab vs. 248 HP / 252+ Def Lickilicky: 150-178 (35.4 - 42%) -- guaranteed 3HKO(Metagrossite)
252+ Atk Adaptability Marshadow Close Combat vs. 248 HP / 252+ Def Lickilicky: 196-232 (46.3 - 54.8%) -- 60.5% chance to 2HKO(Lucarionite)
252+ Atk Marshadow Poison Jab vs. 248 HP / 252+ Def Lickilicky: 132-156 (31.2 - 36.8%) -- 70.1% chance to 3HKO(Lucarionite)
Lickilicky gets some coverage moves to hit Marshadow, but mainly the wish stall lets it beat Marshadow. Using Toxic or Shadow Ball/Zen Headbutt it can beat it.
252+ Atk Tough Claws Marshadow Spectral Thief vs. 248 HP / 252+ Def Heatran: 118-141 (30.6 - 36.6%) -- 62.7% chance to 3HKO(Metagrossite)
252+ Atk Tough Claws Marshadow Poison Jab vs. 248 HP / 252+ Def Heatran: 142-168 (36.8 - 43.6%) -- guaranteed 3HKO(Metagrossite)
0 SpA Heatran Fire Blast vs. 0 HP / 0 SpD Marshadow: 156-184 (48.5 - 57.3%) -- 93.4% chance to 2HKO(Metagrossite)
252+ Atk Adaptability Marshadow Spectral Thief vs. 248 HP / 252+ Def Heatran: 138-164 (35.8 - 42.5%) -- guaranteed 3HKO(Lucarionite)
252+ Atk Marshadow Poison Jab vs. 248 HP / 252+ Def Heatran: 124-146 (32.2 - 37.9%) -- 92.6% chance to 3HKO(Lucarionite)
0 SpA Heatran Fire Blast vs. 0 HP / 0 SpD Marshadow: 184-217 (57.3 - 67.6%) -- guaranteed 2HKO(Lucarionite)
So Heatran can beat it assuming it hits 2 fire blasts which wouldn't happen with my luck :/ but it still beats it.
252+ Atk Tough Claws Marshadow Close Combat vs. 248 HP / 252+ Def Umbreon: 156-184 (39.6 - 46.8%) -- guaranteed 3HKO(Metagrossite)
252+ Atk Tough Claws Marshadow Poison Jab vs. 248 HP / 252+ Def Umbreon: 138-164 (35.1 - 41.7%) -- guaranteed 3HKO(Metagrossite)
0- Atk Umbreon Foul Play vs. 0 HP / 0 Def Marshadow: 175-207 (54.5 - 64.4%) -- guaranteed 2HKO(Metagrossite)
252+ Atk Adaptability Marshadow Close Combat vs. 248 HP / 252+ Def Umbreon: 180-214 (45.8 - 54.4%) -- 53.1% chance to 2HKO(Lucarionite)
252+ Atk Marshadow Poison Jab vs. 248 HP / 252+ Def Umbreon: 122-144 (31 - 36.6%) -- 61.7% chance to 3HKO(Lucarionite)
0- Atk Umbreon Foul Play vs. 0 HP / 0 Def Marshadow: 240-283 (74.7 - 88.1%) -- guaranteed 2HKO(Lucarionite)
So as you can see, Marshadow has counters. You just have to look for them. Which is why I'm voting
NO BAN.