We've datamined a quest coming on the big Mercenaries patch that will celebrate the launch of the new mode with free gifts for everyone!

Upon logging into the game when Hearthstone Mercenaries launches on October 12, a quest completion popup will appear and grant you 2 Free Mercenary Card Packs.

Quote From Out of Cards

Mercenaries Launch Celebration Reward

Here's two free Mercenaries Packs for you!

No further details are available at this time. Keep in mind that datamined information is subject to change until officially announced by Blizzard.