Hello sir, 

I am done my darkrai quest in previous year. and now i am doing mega gengar quest but i cant enter in the door in usion mount . when i interact with npc and trying to enter the door its say the strange power puss you  back. Please do needful. I want to catch lugia shedow form .

 

Hey @Jaydivakar, you probably didnt defeat Umbra in the Dark Realm during the Darkrai quest as that was an optional thing to do.

You would need to go back to the Dark Realm and do the quest once more, make sure to beat Umbra and recruit every pokemon during that quest. In order to go back to the Dark Realm you need to defeat the Horon boss in the Dark Valley, which is north of Necropolis. Once you beat that boss without one of your pokemon fainting, Alucard will respawn in his castle. Go battle him while having a pokemon with the move Miracle Eye in your party and you will get send to the Dark Realm again.