Hideshi Hino and Kazuo Umezu are classic horror manga. I read Nijigahara Holograph by Inio Asano a while back and enjoyed it very much, though it's less morbid and more space/time weirdness.
General suggestions of surreal and weird comics that might not be outright horror would be Charles Burns' "Black Hole", Chester Brown's "Ed the Happy Clown" (a bit goofy and non-sequitur but at the same time eerie and disturbing), Max's "The Extended Dream of Mr. D", Max Andersson's "Pixy" ("Bosnian Flat Dog" is good too), Lorenzo Mattotti's "Fires"... Tardi's WW1 comics, not surrealistic/existential in that sense but they are a very grim and haunting read.
You may also enjoy some Vertigo titles like "Shade the Changing Man", "Sandman" and Alan Moore's "Swamp Thing".
This is all off the top of my head, mostly well-known titles so apologies if you already know these.
