Vexing Scuttler

Emerge lets a creature's printed cost lie to you, and this Crab leans on the gap harder than most. The on the card is the number nobody pays: the emerge cost of reads the mana value of whatever creature you sacrifice and subtracts it, so the real price slides down fast when you feed it something fat. A five-drop on the way out turns this into a two-mana spell. The recursion rides on the cast trigger, which fires when the spell hits the stack rather than when the body resolves, so the instant or sorcery you pull back is already in hand before the Crab has even arrived (and before an opponent can answer the creature to deny the value). That timing is the reason to build around it: the rebuy happens whether or not the 4/5 sticks. The creature is incidental, just large enough to block and survive while the engine does the work. As a colorless creature with a blue emerge cost, it sits in an odd seam: it wants the Eldrazi shell's appetite for sacrifice fodder, but the card it rebuys is a control deck's, a counterspell or burn spell cast a second time. It plays less like an Eldrazi than like a sacrifice payoff that happens to recur your best instant or sorcery.