First time doing a guide but I will be sure to make this as best as I can ^^
Requirements: Darkrai Quest done, have the Mega Bracelet
(Link to Darkrai quest here: https://pokemonrevolution.net/forum/topic/138192-2019-halloween-event-darkrai-quest/)
When you finish the Darkrai quest, get teleported out of the Dark Realm and catch him, you start the quest by going to this house:
Inside you will talk to a Spirit-Sealer, and she will tell you about the evil spirit. It has gotten weakened - was that you?
Now, you go inside the entry to the chamber at the top. A force drags you in and you should see this -
(Sorry the picture was too big :p)
Go to the top portal and enter the Nightmare Battlefield.
On the left side of the entrance the pokemon/spirits from the Dark Realm last time when you beat Darkrai wait for you as you recall them to your side.
Something to note is their IV's are all set to 01, but I think so are the opponents pokemon as well so... yeah. Another thing to note is that they're all now lvl 100 and it doesn't depend on how much you trained them previously in the Dark Realm.
Now, I just beat this but warning - this is very hard and it might take a combination of strategy, google, and luck.
So I suggest you search up the weaknesses but this guide is a guide, so -
Honchkrow is weak against Clefairy and Musharna - I used Dazzling Gleam as it hit other pokemon (including my own team sadly) as well.
Weavile is weak against Marowak's Earthquake - and it also hits other pokemon surrounding it. (including your own team).
Scrafty is weak against Clefairy and Musharna (but Arcanine's Flare Blitz works good as well), Dazzling Gleam or any other fairy type move is good.
Dusknoir is weak against Raticate-Alolan's Crunch, when it dies (as it's probably the weakest pokemon here) use Arcanine or any other pokemon you think is good as I couldn't find any other type good against Dusknoir for some reason.
Cofagrigus - I had the same problem as with Dusknoir, it's only really bad weakness is Ghost/Dark so I used Arcanine's Flare Blitz (as well with a combination if Dazzling Gleam was there).
Drifblim is weak Marowak's Earthquake or Arcanine's Flare Blitz, as I couldn't find too many counters.
The sad/unfair thing is you only get 5 pokemon but the good thing is that they counter some pokemon.
Some tips.
Pokemon that are dying - I suggest you recall them and if HP charge is open use it. If it has enemies keep the pokes in your slots.
Charges - It depends on what you choose to do - keep fighting or use a spdef charge to gain, well, spdef.
After you win the game on Nightmare Battlefield you will have to go and face Umbra. He has a Mega Gengar so beware.
I had only Marowak on my side, also the Pokemon/spirits return to full health when you face Umbra (I think).
However, doing my research - Gengar/its type is weak against Rock/Ground, and as Gengar evolved into Mega Gengar, I tested my luck - I would need it to one hit it...
Mega Gengar used a move (I'm not sure what) that almost killed my Marowak. I used Earthquake and luckily, it one-hit Mega Gengar.
Umbra will yell out "No! You weren't supposed to beat me! I only battle losers!" and you get teleported out of the Nightmare Battlefield.
You talk to the spirit-sealer and she says thank you. To get gengarite you go above this area, you will see a pokeball:
You have obtained Gengarite. The spirit-sealer will say she needed a trust-worthy trainer and a good one to give it to. She thinks it's safe in your hands.
This is the end of my guide, I hope it was helpful - especially to those who needed it because I couldn't find a single good guide.
: Mega Halloween Event Gengar