Eldest daughter of Oceanus and mother of Nike (4)

I believe the answer is:

styx

(Other definitions for styx that I've seen before include "River separating Earth from Hades" , "Mythological river in Hades" , "River Charon rowed on" , "The last river one crosses" , "River of hell" .)

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

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