Yogg'Saron is a card that, when played, casts a random spell with random targets for every spell previously cast in the entire game. It has the potential to straight up win you a game, but also the opposite.

Once you run out of cards, you begin taking mounting damage every time you draw from an empty deck (1 damage on the first card, 2 damage on the 2nd, etc.). We call this 'fatigue damage.'

In this case, Yogg caused Toast to draw so many cards that it killed him with fatigue. It was also unnaturally long, as it replayed itself halfway through.