A 4-mana 6/6 that always makes you sacrifice a creature when it enters isnt worth it except in decks with good sacrifice fodder
P/T Number 65 Language English Legal Formats Commander, Legacy, Modern, Pauper, Vintage Not Legal Formats Alchemy, Brawl, Competitive Brawl, Historic, Pioneer, Standard, Timeless Rulings (2/1/2007) When this spell is cast, its "when you cast" ability triggers and goes on the stack on top of it
Why not just play Notion Thief, you ask
Whenever one or more +1/+1 counters are put on Emperor of Bones, put a creature card exiled with Emperor of Bones onto the battlefield under your control with a finality counter on it
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Hedron Detonator, Academy Manufactor, Tireless Tracker, Rashmi and Ragavan, Shimmer Dragon, Reverse Engineer, Thoughtcast, Dance with Calamity, Thopter Spy Network, Aid from the Cowl, Stroke of Genius 11 card draw