So far the spoiler season has been notable for its relative lack of random effects. The fourth Old God, Yogg-Saron, has been revealed today, and has changed that to some degree. Blizzard have also revealed the lore behind Yogg-Saron. This brings the total number of cards revealed so far to fifty three.
The fourth of the Old Gods, Yogg-Saron, does not seem to be very powerful. There are quite a few cards that do not have targets that can benefit the player, such as and , and as such, Yogg-Saron will usually have a net positive effect. However, in a deck where you've battled to drop your ten mana card, it is likely you will want more consistency than this provides. With such an unusual effect, it will still require testing before we can write it off.
Spreading Madness does not seem like it will see constructed play. The amount of damage it does to the opposing player does not seem to be enough to merit inclusion in an aggressive deck, and there are cards such as and for more reliable removal options.As well as the two cards, Blizzard have also released a video explaining the lore behind Yogg-Saron.