The speaker feels contaminated by experience. To "reek" of what they have seen is to say that sight doesn't simply pass through you; it clings, like smoke in clothing, and other people can smell it. The word "surely," repeated twice, sounds less like confidence than like a grim admission. There's no doubt left to argue with.
The last two lines are the strangest. The speaker says the words they "spew" are "the shapes they shy at." Spew suggests speech that comes out involuntarily, almost like sickness. And the "shapes" might be the very things the speaker has seen, the images they flinch from. So the act of speaking, which might be expected to release or explain what haunts them, only gives it form again. The poem seems to suggest that the speaker can't escape what they've witnessed, even by putting it into language.
The mood is uneasy, tight and nocturnal, which fits the title's reference to night. It feels like a confession made reluctantly.