Creatures : from appearance alone , easily creatures near Thriller Bark ( potentially Umibozu )
They would completely & utterly dwarf biggest measurable character in One Piece so far ( San Juan Wolf , 180 meters tall ) , plus they have red eyes ( both in Anime & Coloured Manga ) , they seem to have dark colour too which would enhance their scary looking
Object : not sure ? Thriller Bark and Mihawk's castle sure are creepy though
Also Brook's former ship that used to sail around Thriller Bark , in Florian Triangle
Character : Kaido ( if you know his strength )
If you don't know anyone's strength then Oars , because he is the biggest + scariest looking character ( 67 meters tall )
Island : well Thriller Bark can be here , Mihawk's island Kuraigana also
Power : Goro - Goro No Mi , Thunders from the sky ( he can probably make this with Awakening ) , how do you stand up to that even
Magu Magu , Pika Pika with Awakening & Light Speed , Yami Yami are scary as well