Mythical river of the underworld (4)

I believe the answer is:

styx

(Other definitions for styx that I've seen before include "Mythological river ferried by Charon" , "Underworld river; sounds like remote area" , "Hell's river" , "Underworld river with ferryman Charon" , "River crossed at death" .)

I've seen this clue in The New York Times and the LA Times.

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