Blizzard have released a video and blog entry explaining some of the lore behind Y'Sharrj. Y'Sharrj is the third Old God to be confirmed to be coming to Hearthstone, alongside C'Thun and N'Zoth. The two new cards revealed here bring the total revealed so far to forty two.
Y'Sharrj was the most powerful of the four Old Gods to enter Azeroth, and as such we should expect the card to be incredibly powerful.
Although not particularly overpowered at first glance, there could be ways to use Y'Shaarj as a win condition. It automatically brings one card into play the turn you play it, but importantly can also be fetched from your deck by newly revealed Ancient Harbinger. Given the interaction between Ancient Harbinger and the other Gods, as well as Y'Shaarj and the other Gods, it seems likely that players will try to make a God deck sooner rather than later. The lore says that Y'Shaarj was able to raise an army, and this card certainly has the power to do that!
Steward of Darkshire, Cabalist's Tome, and Skeram Cultist: .Mark of Y'Shaarj, Shadow Word: Horror, and Scaled Nightmare: .Ragnaros Lightlord, Mukla Tyrant of the Vale, and Cult Apothecary: .Boogeymonster, Renounce Darkness, Xaril, Infest: .Undercity Huckster, Tentacles for Arms, Spawn of N'Zoth: .N'Zoth the Corrupter, and lore: .