Being able to cast two spells a turn should be achievable in most decks, and regardless of whether youre ramping into an expensive or early commander, youll want to be playing some amount of ramp to make this happen
Once Brotherhood Spy's ability has resolved, removing the legendary Assassin (or Assassins) you control from the battlefield or causing them to lose the legendary supertype or Assassin subtype won't cause Brotherhood Spy's ability to stop applying for that turn
This amazing white creature can also get additional value out of cards like Uro, Titan of Nature's Wrath each time it attacks
If any are here (they aren't) they won't make the cut here either
As above, first strike and double strike only work during the combat step
The real power comes from Shadowfaxs ability to take a creature card from your hand with lower power than Shadowfaxs four and put it directly into play tapped and attacking when Shadowfax attacks