AtD translation: by whatever terrible singularity in the smooth flow of Time had opened to them

[Mike Jing]

P555.7-14   Miles, taken by a desolate illumination, reached out his hand,
and Thorn, seeing his intention, flinched and backed away, and in the
instant Miles understood that there had been no miracle, no brilliant
technical coup, in fact no “time travel” at all—that the presence in this
world of Thorn and his people had been owing only to some chance blundering
upon a shortcut through unknown topographies of Time, enabled somehow by
whatever was to happen here, in this part of West Flanders where they
stood, by whatever terrible singularity in the smooth flow of Time had
opened to them.

I'm having trouble parsing the last part of this sentence. Is there a
relative pronoun missing or simply implied?

I assume the word "by" corresponds to "enabled" from earlier in the text.