The River Styx is from Greek mythology. It was the river of life and death, and stepping in it caused you to be immune to attacks -- but only the part of your body that was submerged. If any part of your body wasn't in the water, you could be killed. Achilles was dipped in the river as a baby by his mother, but he was held by his heel. Eventually, in a war, he was killed by a poison arrow in his heel. His heel was his only weak spot -- Achilles' heel.
The River Styx is from Greek mythology. It was the river of life and death, and stepping in it caused you to be immune to attacks -- but only the part of your body that was submerged. If any part of your body wasn't in the water, you could be killed. Achilles was dipped in the river as a baby by his mother, but he was held by his heel. Eventually, in a war, he was killed by a poison arrow in his heel. His heel was his only weak spot -- Achilles' heel.