Serra Aviary

The World type is the load-bearing detail here, and it is the line the oracle wording makes easy to skim past. World is one of Magic's oldest balancing valves: only one World permanent can exist at a time, so when a second one enters, a state-based action puts all but the newest into their owners' graveyards. That is not a destroy effect, which matters more than it looks; it bypasses indestructible entirely, sending the older World enchantment to the yard regardless of protection. The buff itself is unconditional and symmetric: every creature with flying gets bigger, yours or anyone else's, so the static team-wide bonus was strong enough, in mid-90s thinking, to need the World constraint as a counterweight.

What that produces is a flying anthem with an unusual self-replacement clause baked into its type line rather than its text. Two players can each run a World enchantment, but they cannot both keep one on the battlefield; the older falls away to the graveyard the instant the newer resolves. Because the +1/+1 is symmetric, an opponent already in the air has no reason to break the parity: your Aviary pumps their flyers too, and replacing it with their own copy just keeps an identical buff on the board from a different source. The real tension is that the slot is shared and singular, a snapshot of a design era when Wizards leaned on the World rule to police static effects before largely shelving the mechanic.