It's hard to argue that it shouldn't be here, right
The cost reduction applies only to generic mana in the total cost of black spells you cast
Shadow, Mysterious Assassin Its time to break out the Whispersilk Cloak, because we really want to connect with Shadow, Mysterious Assassin
Granted, you need to invest some mana, but green decks are interested in a 2/1 that has monstrous that can be paid in small installments
It has its own restrictions, but it will still find its place in the meta
However, once you've run through a couple of practice rounds, you'll be able to get the rules down pat, and you can always check back in with Pokemon's official tutorial and rulebook to make sure that you're playing it right every time