It works with cards that double the amount of counters on permanents (any counters, not just +1/+1 counters)
4 Aurora, Dreaming Guardian (Enchanted) - $174.96 Set One, The First Chapter Aurora, Dreaming Guardian, is a card that if you set your board upright, creates a nearly impenetrable wall of protection for her allies on the field
Each bracket has a set of deck-building conditions: if a deck meets those conditions, it sits in that bracket
In rare cases, the mutated creatures second creature type may not even be discernable based on its appearance in card art
This causes the Day of the Dragons to leave the battlefield since its a creature, which then sacrifices all dragons you control and returns the exiled cards (notably Opalescence and Day of the Dragons) back to the battlefield
(One of my favorite plays, BTW.) Sure, the other Morphs force you to make decisions as well, but with the Walker they always seem to be much closer than with the others