I have never read one of Dan Simmons horror novels, although I have read most of his science fiction. As you may be able to tell from the title, The Terror is a horror novel. The name itself refers to one of the ships in the story, the HMS Terror, whose crew makes up the main characters. The novel is the story of the true quest of the HMS Terror and the HMS Erebus, led by Sir John Franklin, to find the fabled Northwest Passage. It is true that the ships were lost and the crews never heard from again. Simmons gives life to them here, spelling out the crew members final days in the arctic. Only in his version, it is not simply cold and lack of resources that kills them. There is also a monster stalking them on the ice.
The book is a page turner at times, definitely scary at times, and very graphic in the various perils that befall the poor crew. The graphic descriptions are not reserved only for those who are killed by the monsters, but also those who are killed by the various diseases that afflict the crew members. So much of the book is taken up with these descriptions, it seems that he should just kill them all off and be done with it. Reading it, you basically decide that that would be better than continuing to read about their deaths in the innumerable permutations that Simmons comes up with.
When crew member, the captain of the Terror, Francis Crozier, experiences a different fate. But it takes so long to get there, that Simmons leaves only a few chapters to tell about it. It would have been preferable to hear less about the drama and despair and death experienced, and learn more of Crozier's experience. It gives the book life and hope, but maybe in a book titled The Terror, life and hope are less important than death.