Definitely Batman: Long Halloween.
What 4GRANted said
The Halloween special that came out last year was amazing! I hope this years Cursed Cavalcade is the same style.
Anything swamp thing too
If you are just looking for recs from the app, these have at least some horror elements.
Forbidden Tales of Dark Mansion
House of Secrets
Penguin: Pain & Prejudice
The Dark Mansion of Forbidden Love
Frankenstein and the Agents of S.H.A.D.E… It’s woefully underrated.
In the comic library here is the 50 page special, “Batman: Madness - A Legends of the Dark Knight Halloween Special”.
The Sandman has its creepy moments