Styx was an action maze game published by Bug-Byte Software in 1983. It was the first ZX Spectrum game written by Matthew Smith, and the first of his three-game contract with the company. He went on to write Manic Miner in the same year, as well as a sprite designer utility for Adrian Sherwin to use in developing The Birds and the Bees.
Sinclair User described the game as "reasonably lively", whilst CRASH said the game "would not be interesting if it weren’t for the fact that it’s not easy."
Source: Wikipedia, "Styx," available under the CC-BY-SA License.