adverb, adverb taking the 'to' particle
sound of a ghost appearing
Onomatopoeic phrase evoking a spooky, eerie atmosphere, often used in manga, anime, or ghost stories when a ghost or yōkai appears.
お化けが出るとき、ヒュードロドロという音がした。
When the ghost appeared, there was a spooky 'hyuu-dorodoro' sound.