He's a zombie

When Sathanus was about to just kill Basilisk, something happened that shook each and every spectator in the stadium.

The ground was shaking badly as an earthquake. Sathanus was bamboozled because something was happening, of which he had no idea, and when he was just trying to figure out what was going on, a giant snake came out of the ground, ripping everything apart. 

It was "Slyory" Kai's second most powerful serpent.

He was quick and because of his agility; he was able to pierce his fangs in one of Sathanus's leg.

"AAARRGGHH!!!" Sathanus, who was about to slay Basilisk, was dragged far away from him by Slyory. The wound he was given by Slyory was fatal it was enough to bring tears in his eyes.

But it was expected, because imagine being bitten by a serpent with a bus sized head.

He was screaming painfully as he'd die of the pain.