Good afternoon, I would like to know if it is possible to restart the Hallow quest, since I cannot capture darkai, thank you very much

Hey, if you want to go back to the Dark Realm 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. Defeat Alucard with a pokemon that has the move Miracle Eye on your party and you will find yourself on Dark Realm again.