If Fecund Greenshell's second ability stops applying (because Fecund Greenshell leaves the battlefield or loses its abilities or because you no longer control ten or more lands), then any creatures that needed the toughness bonus to stay alive will die
What you do is Brainstorm your business spell back on top of your deck, put the cantrip on the stack, and in response crack the Lions Eye Diamond(s)
It is probably useful to count Ponder and Brainstorm as simple cantrips as well
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The problem we ran into during playtests was players didn't understand why those three things went together
The simplest way to abuse this is to ram Dark Necrofear against a stronger monster