The first Immunity challenge was a race right after the tribes were formed....each race the winner would keep his tribe safe and the last one to the finish from the losing tribe would be eliminated...on the losing tribe whoever was the last standing from their tribe would win individual immunity for the first tribal council and a clue to the hidden immunity idol.
Winning Tribe-The Argonauts
Final Race-Argus and Io were the last 2 people in the race.
Indivdual Immunity winner-Io
Clue to hidden immunity idols-Argus and Io.
The reward for winning tribe-Flint.