If Tree of Perdition isnt on the battlefield when the ability resolves, the exchange cant happen and the ability will have no effect
Number 100 Language English Legal Formats Commander, Legacy, Vintage Not Legal Formats Alchemy, Brawl, Competitive Brawl, Historic, Modern, Pauper, Pioneer, Standard, Timeless Rulings (9/1/2023) Defense of the Heart's ability will check if an opponent controls three or more creatures as your upkeep begins
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This legendary restriction is in place mostly for flavor reasons, but also helps to balance these powerful permanents
And while, yes, it doesnt necessarily do anything until you have a creature with power five or greater, once you meet that threshold it provides consistent benefits to you and your board