Harrowing Pack = Expansion 3 Pack. Its not a limited time pack. Id recommend you keep buying Master Packs until you develop a good card base. Failing that, Id probably suggest the new Expansion 3 Pack (Harrowing). Expansion 2 has a lot of special use cards that you wont benefit enough from. Expansion 1, well most of its good cards are unlockable or unlockable only.
To get an idea of what pack you want, you may want to click the Collection button at the top of the game (right under how much light you have). This allows you to see whats in each expansion.