Previously I had completed the main quest at the Halloween event and had obtained the clothes according to the instructions, but I couldn't start the Mega Gengar quest at this event, please help

Hey, thats not a bug.

To proceed you need to beat Umbra (the Mega-Gengar) within the Dark Realm which you find yourself in during the Darkrai quest.

I already completed the darkrai quest before, how do I go to the Dark Realm again?

Edited November 1, 20241 yr by Barakbar155

If you want to go back to the Dark Realm you need to first defeat the Horon boss north of Necropolis in the Dark Valley, make sure to beat him without one of your pokemon fainting. Then you will be able to re-enter Alucards castle where you can access the Dark Realm again to repeat the quest.

like your message, I have defeated Horon without any of my pokemon dying, so I got the new rapidash mount, and after that, when I entered Alucards castle and defeated Alucard without any of my pokemon dying, I still couldn't enter the Dark Realm. left or right is the same. is there something wrong?

As before you need to beat Alucard while having a pokemon with the move Miracle Eye in your party.
If that does not work you need to beat Horon again after the cooldown, as it either rewards you with the Rapidash Mount or lets you enter the Dark Realm again.

Edited November 1, 20241 yr by Norex