River that made Achilles (mostly) invincible (4)

I believe the answer is:

styx

(Other definitions for styx that I've seen before include "River separating Earth from Hades" , "River surrounding Hades" , "Charon's 26 carried dead souls across this river" , "River crossed at death" , "Infernal runner" .)

I've seen this clue in the LA Times.

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