“Redhorn” is not the correct answer either, both answers answer only part of the riddle. The language in the riddle indicates that this is a very evil site, likely inside Mordor ? What about “Barad-dûr” ?
- Ithil Pass,
- Black Gate, or another gate?
- Nasty history
Caradhras was also the logical answer for me. Maybe try its dwarvish name, Barazinbar ?
Does the riddle say whether it expects only one or two words as an answer ? If two, maybe Redhorn Pass or Redhorn Gate would work ?
Other than that I can only think of one other “pass of dire reputation” : Cirith Ungol. The only dreaded spire around is Minas Morgul. But I don’t know how it matches with “glowing with blood”…
Mordor sky is glowing with blood.