The reveal yesterday of Yogg-Saron, Hope's End threw out many questions regarding the rules of the card. Redditor gamingdude295 has compiled a list of relevant questions and answers. Also on Reddit, adambard has created a Yogg-Saron simulator!
Yogg-Saron creates spells at random from all collectible spells in the format you are playing. These spells can target either hero and any minion, including itself, however if there is no legal target available for a given spell, then that spell will not be cast. If the chosen spell has an associated choice, the choice will be made at random. If Yogg-Saron kills itself, the Battlecry will still continue to resolve, but if it kills a hero, the game will end immediately.
Spells follow the rules as if your hero had cast them. This means that will give the opponent a mana crystal, and will discard your cards. Despite this, Yogg-Saron is still casting the spells, so , , and will not trigger. (Although if YOU somehow cast a spell after Yogg-Saron casts Lock and Load, then that particular spell WILL trigger it.) Minions are also not effected. This means that does not get buffed from the spells, and will not trigger. Hallazeal the Ascended DOES trigger however due to the slightly different wording of "your spells" rather than "you cast a spell".Some odds and ends:- If you happen to have any mana available, then Forbidden spells will use that mana.- will result in excess mana being added to your hand.- If you only played Yogg-Saron, Combos will NOT activate. If you play Yogg-Saron after another card, the combos will be active. - Secrets played by do NOT count towards Yogg's total spell count. The Reddit itself contains all of the sources for the information.At the time of writing, the page is down. That should become available as the high traffic to the page dies down.