Yogg-Saron went through a variety of changes before making his debut in Hearthstone's Whispers of The Old Gods expansion.
C'Thun may have been the poster child for Hearthstone's Whispers of The Old Gods expansion, but the star is most definitely Yogg-Saron - who randomly casts random spells at random targets, leading to some hilarious comebacks, and devastating misplays. Blizzard had previously said that the card was easily the most complicated card they have ever released, but before its spell-slinging form we see today, it went through a variety of iterations.
In an exclusive interview with designers Mike Donais and Peter Whalen, Shacknews reveals some unused Yogg-Saron concepts. Check them out (in chronological order) below:
As you can clearly see, the initial vision for this Old God was quite different. The original version of the creature gave you just 15 seconds to make your turns, a kind of "self-only Nozdormu effect", but had possibly the most powerful effect in the entire game. Mike Donais said some combinations were "absolutely ridiculous," and as a result it didn't take long to change this design into something a little more reasonable. In particular, he said, playtesters would use Mage decks with Pyroblast and Arcane Intellect to deal massive damage and keep their hands full for more burn spells.
The second design was even crazier, giving players a massive starting advantage with the disadvantage of never being able to go over 4 mana crystals. Donais said this was a cool concept, and believe it or not, was actually quite balanced (Yogg players won roughly 50% of their games), but it was really boring for both players. The Yogg player simply stacked his deck with 4 cost minions, while the opposing player just had to hold out until turn 5 and then win.
The third iteration briefly revisited the "15 seconds a turn" idea. This time, it seemed balanced, but the problem was a technical one. "We had some problems with that, because the phone interface is a bit slower, and also if you had just a minor lag on your Internet service or if some cards don't interact perfectly," Donais said. "We'd have to really engineer turn timers so they were perfect. We didn't have the resources to do that and weren't sure it would be worth doing the card even if we had that."
Finally, the fourth version of Yogg-Saron had all the pieces we see in the live version - randomly casting a copy of each of the spells the player had cast. Unfortunately, this was just a little too powerful, as players could use it to plan out extremely powerful combos.
So, the "casino" factor was added in the second-to-last design, and the final redesign simply saw a minor tweak in stats, bringing us to the Yogg-Saron we all love and worship today.