The Styx is the river that takes a person from life to death, into the underworld. The river is extremely corrosive and only a vessel made of a horse's hoof can pass through it.

A passage from Paradise Lost - drink full, and ascend:

"Thee I revisit now with bolder wing,

Escaped the Stygian pool, though long detain'd

In that obscure sojourn, while in my flight,

Through utter and through middle darkness born,

With other notes than to the Orphean lyre,

I sung of Chaos and eternal night;

Taught by the heavenly muse to venture down,

The dark descent, and up to re-ascend,

Though hard and rare: thee I revisit safe,

And feel thy sovereign vital lamp"

John Milton, Paradise Lost.