For Mirror Box, that rule is the legendary rule that says you're only allowed one copy of a legendary creature card on your battlefield at the same time
Alesha, Who Smiles at Death Alesha, Who Smiles at Death puts a spicy spin on reanimation decks by putting small, impactful creatures like Master of Cruelties and Unstoppable Slasher into play
Unfortunately, as Sottosanti notes, momentum continued to drift away from purple during the design process, and it was ultimately left on the cutting room floor
Set Number 90 Language Franais Legal Formats Commander, Legacy, Modern, Pioneer, Vintage Not Legal Formats Alchemy, Brawl, Competitive Brawl, Historic, Pauper, Standard, Timeless Rulings (6/22/2015) Check to see if there are two or more instant and/or sorcery cards in your graveyard as the spell resolves to determine whether the spell mastery ability applies