who or what is derivation of the term "soot styx"?

Question

in a crossword puzzle it gives a quote of or by someone or from some referencen and it may have to do with stain or mark denoting burning or just how hot is the river STYX

Answer 1

That's a new one for me. I don't think that the Styx was a hot river. There are two other rivers in the underworld according to mythology, and one of them is boiling blood, but it's not the Styx.

Answer 2

There are five rivers in Hades--Acheron, Lethe, Cocythus, Pyriphlegethon and Styx. The Styx is said to be firey hot, or boiling.
Could the word possibly be charred?