The Darkrai Quest is one out of many additional, not story related, Legendary Pokémon quests and is the only way to catch the Mythical Pokémon Darkrai.
Accomplishment of the quest’s requirements
In order to move forward with the Darkrai quest, you need to complete the first part of the Halloween (quest). It is recommended to have 6 100 Level Pokémon in your party, to not struggle against the questline’s NPCs.
You’ll need the following to complete the quest:
— Pokemon with Miracle Eye
— Pokemon with Foresight
— Boss team
After finishing the Halloween (quest) you move north again to the Unison Mountain South Summit. There you enter the Shrine Interior. At the end of the room is an entrance but when you try to enter you’re pushed back. Talking to Spirit-Sealer Selina reveals clues about what to do next.
Move southeast and enter the Unison Mountain. The spirits are now gone so you can move freely. Move through the East Summit and into Howling woods. From there go east to Dreadtown and into Vlad’s house. North east of Vlad himself you’ll notice a hole in the wall. Move close to it from the right and you can go through it. You now entered Spooky Path, move through it to Uncanny Path. Move through that as well until you reach Necropolis. Locate and enter the Alucard Castle to the north.
Inside the castle 2 kids are blocking the 2 stairways. To proceed you need to have on you a pokemon with Foresight. You need to search around until you find 3 invisible pokemon and battle them at a specific order. Then the kid on the left is gone and you enter another room. Inside you follow the path and battle invisible pokemon PMCs every time you stop. When all 4 pokemon are beat your success will be recognised by the ghost at the end of the room. Now talking to the 2nd kid makes it disappear as well. Enter the room behind it and follow the teleportation path while trying to find and beat 6 invisible pokemon until you reach the ghost Alfred. He will let you pass after you have defeated all hidden pokemon. Inside the next room you have to battle Alucard Master of Deception. After you defeat him you he will send you back to the pokemon center unless you have a pokemon with the move Miracle Eye. If you do you’ll learn that he is in fact Darkrai in disguise. Fight him but prepare to be beaten badly, his EVs are extremely high.
Edge of Darkness
After your defeat you’re sent to the Edge of Darkness from where you can escape by stepping on the right light and you’re sent to the Dark Realm.
In that realm you have to team up with Munna (where you start) and Growlith at the North West corner of that map. Growlith is attacked by 4 gastly. There is a repel nearby, grab it and when you get close you’ll be asked if you want to use it. That way you can fight fewer gastly. Head towards the North East corner and move through to the Dark Realm Castle Outside.
Dark Realm Castle
The place is lurking with Murkrow who will attack you if they spot you. If you lose you’re sent back to the Edge of Darkness and start over. On the right is the cave exit, so ignore it for now and on the left is the castle you’re looking for. Inside there are several golbat you can try to avoid or choose to fight to grind your pokemon.
Here you can recruit a Clefairy by a series of interactions presented below:
— Entei (Volcanoes Erupt)
— Dragonite (Jealousy and Left)
— Ho-Oh (Sky)
Next go through the bottom entrance, it will lead you to a room with cells, beds and offices. Follow the clues as in the photo once more to ally yourself with an alolan rattata:
In the same room you get your first chance to heal your pokemon. Interact with the computer in the office room and the barrier next room will be deactivated. Now you can interact with Audino and heal your pokemon whenever you need to.
Next you enter the cave where you find a cubone in the top right corner. Again you must perform a sequence of actions to ally it. But first you move forward and go north to the Inner cave. There your fairy type pokemon allows you to remain and fight a shiny ghastly and a mega Gengar. After defeating it a tutor will appear that can teach your clefairy softboiled. Now go back to the previous room to ally the cubone. You can see the locations of the items/pokemon you have to interact with in the picture below:
In the middle of the cave you’ll be given the opportunity to fight for a fire stone and evolve your growlith. But it is suggested that you first level up your growlith beyond lvl 40. Next go back inside the castle and this time go to the top of the staircase and to the right you’ll find another entrance. Going in you’re faced by Nightingale and his minions. You’ll have to fight 1 battle vs 6 pokemon (3 Murkrow and 3 Purloin, 31-33lvl) after you win you can claim a Moon stone for your clefairy a Thunder wave and black glasses.
Head back to the dark realm and all the way south until you reach the Magus Castle Outside. There a haunter will be standing in front of the entrance to the castle blocking some soul. Battle the haunter lvl 30 and the soul will prove to be a tutor for the move Reversal. Upon entering the castle a bunch of Mismagius are guarding the floor. On the next floor 2 Pawniards are guarding there too. On that floor you’ll also find an absol guarding the entrance to an Audino and your new way to heal. 2 more [Pawniards]] guard the 3rd floor as well. And on the 4th floor is the castle summit where darkrai is, guarded by several pawniard and their evolved form. After you defeat them he will spawn 2 spiritomb, one of them shiny.
It is suggested to procced only if you have clefairy 55 and grow Growlith beyond 50 and evolve it with the fire stone. Careful, if you leave the summit area it is restored to default. But that can work to your advantage to train your growlith faster. Finally Darkrai will send out 2 out of these 5: Torterra, Infernape, Empoleon, Togekiss or Garchomp
After beating them you immediately fight darkrai at lvl 55. For now just beat him with your clefairy. After the battle you can go back get your sync pokemon and go catch it! Good luck!