The Bridge of Khalas is in the first Layer of Gehenna, the only known place in the entire layer of crossing the turbulent River Styx without resorting to teleportation or flight. Often, even the marraenoloths can't manage it, as the Styx in Gehenna is mostly rapids and deadly waterfalls.
It's been the site of uncountable Blood War battles. Whoever holds the bridge - be they Tanar'ri or Baatezu - holds the layer of Khalas.
Source
Hellbound: The Blood War
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