One C'thun and two Beckoner of Evil cards are rewarded after you acquire at least one pack of Whispers of the Old Gods and enter the "Open Packs" screen.
This will remain in place for as long as Whispers of the Old Gods packs are available, presumably, since the stated reason for the promotion is so players can use all the common and rare C'thun synergy cards in the set without having to worry about spending dust or money to get the centerpiece. In addition, the normal versions of these cards are presently uncraftable.
During the launch window of Whispers of the Old Gods, logging in will reward you three free packs of the set, so you will not need to spend any money or in-game gold to get C'thun if you take advantage of the launch promotion. This began on the Whispers of the Old Gods launch date, and the packs are awarded when you log in to the game. An end date has not yet been announced.
Both of these promotions were revealed at the Whispers of the Old Gods announcement press event, according to BlizzPro.
During the Hearthstone live-stream on April 21st, two more promotions were announced:
- Win 2 games of Standard after launch to receive 5 free Whispers of the Old Gods packs
- Win 7 further games of Standard (for a total of nine) to receive a further 5 free Old Gods packs
These promotions show up as "Legendary Quests" in your quest log. If you already had three active daily quests, they replace the quest on which you had the least progress, or the least valuable quest if you had no progress on any of them.