Summary: Shadow Crystals are essentially the in game currency of the Shadow Era. This allows you to purchase booster packs and theme decks. You can buy individual cards by in game currency (golds) but purchased booster packs and theme deck contents i.e. individual cards can be sold; hence, converted into golds. In any event, how do you earn Shadow Crystals?
The following link outlines well. But essentially 3 ways.
Currently available promotion is through Wulven. You can entire your email account associated with Shadow Era account, and immediate gain between 100-200 shadow crystals. You can do this only one time per account though. But if you haven’t, definitely no reason not to.
FREE and PAID but you have to complete different tasks e.g. filling online survey, download online game etc.
Simply, purchase crystals using paypal or other method of payment. Soon we should be able to purchase physical card (booster packs or theme decks) and gain the equivalent price amount of Shadow crystal for digital game use. Essentially, you are getting physical cards for free. No other CCG whether major or minor had ever been this generous in its price scheme (as far as I know).
Supposedly, if your friend or family member purchases Shadow Crystal you get 10% kick back.