The Tentacled One
They're obnoxiously rare in booster packs of the sets that have them (Battle for Zendikar, Oath of the Gatewatch, Kaladesh, Aether Revolt, Amonkhet, Hour of Devastation). I heard the ratio was supposedly around one masterpiece per booster box. So it's either get super lucky or buy from someone else who did. Stores sell that ugly version of Dark Ritual for over $40 apiece.Oh. I don't have that one. I don't even know how to get those Invocation/Masterpiece cards.