There are enough good cards with defender that you can easily scale up your 40 cards to 100 without hurting the power level of your deck too much
What breaks it: any instant-speed removal on Trazyn with the trigger on the stack, or graveyard hate that eats the rock youre copying
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With that in mind, lets look at how all those parts fit together: That hits Nephilim colours, Nephilim creature type, and the core mechanics of each nephilim, now presented on a creature you can build a deck around
The card also reads: Whenever Narset, Enlightened Exile attacks, exile target noncreature, nonland card with mana value less than Narsets power from a graveyard and copy it