He sent his companions to slay the dragon , but they all perished .
On seeking to establish the city , Cadmus required water from a spring guarded by a water - dragon similar to the Lernaean Hydra .
Cadmus sent Cadmus ' companions to slay the dragon , but they all perished .
On seeking to establish the city , Cadmus required water from a spring guarded by a water - dragon similar to the Lernaean Hydra .