Its effectively a 4/4 Eternal Witness, and it doesnt take much effort to give it an additional relevant keyword like flying or trample
An instant is more powerful than a sorcery, but the requirement to attack to net a card is much easier to accomplish than having five different mana values in your graveyard
They've been used in lots of sets so far, and from the looks of it well be seeing more in the future
But even if you do not want to plan your whole deck around Behemoth K, he is just strong card in general and will be included in many Lightning-Decks for sure, especially since you can search him with [10-086C] Aldo, [9-063L] Gabranth, [11-072R] Tyro and another Card in Opus XIII: Crystal Radiance I also like the idea of running Behemoth K in an aggressive Lightning-Deck with Cards that constantly treating your opponent and trying to deal 7 points of damage very quickly
So at worst you have a 6/6 samurai, but at best you hit a living weapon, job select card, For Mirrodin!, Meteor Sword, Embercleave, and the nasty Nazgl Battle-Mace
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