Greetings @Cyrillia
Apologies for the inconvenience encountered.
As mentioned by NarutoUzumaki12 and Norex, you will be able to reset the Darkrai encounter by beating Horon in the Dark valley, located north of Necropolis. The requirement needed for this boss is that you defeat him without having any of your pokemon faint. Once that is done you will be able to return to Alucard Castle and proceed with the quest. This will have different results, depending on where you are in the quest :
Alucard will be respawned if you didn't access the Dark realm yet. Defeating him while having a pokemon with miracle eye will give you access to the Dark realm.
Darkrai will be accessible for you to catch again in Alucard castle if you were in the process of catching it and failed to catch it.
You will need to wait for Horon cooldown if he gives you the Halloween Rapidash mount, which is of 3 days. Fighting him again after this will allow you to continue the quest.
Please let me know if that solution works for you, I'll be waiting for your answer.
Salutations,
Midsommar
