After completing the dark relm quest successfully ,while catching darkai had fented . So is there any 2nd chance to catch darkai.

You have to respawn Alucard in his Castle on the third floor, by defeating the Horon boss in the Dark Valley (north of Necropolis).
Make sure to not let any pokemon of you faint while battling Horon. If you didnt get the Rapidash Mount from Horon yet, you will get rewarded with the mount on beating Horon. You have to wait 3 days for Horon to respawn after that. If you beat him again, Alucard will respawn.
Defeat Alucard with a pokemon that has the move Miracle Eye on your party and you will find yourself on Dark Realm again.

Hi @abir86

Thanks to Norex for clearing it up.
You will indeed need to challenge Horon at Dark Valley, which is North from Necropolis, without having a single Pokemon faint to have another chance in catching Darkrai.

 

I hope this helped you.

This thread will be locked now, please make a new one should you run into any further issues.

 

Have a wonderful day.