Similarly to Jafar, this gives you a way to repeat enter triggers on cards like Giant Cobra
Causing players to turn one of their non-island lands into an island each turn in the form of flood counters, Quicksilver Fountain enables your merfolk with islandwalk across the entire board while also slowly interfering with the mana of any of your opponents who aren't playing blue
Her ability to generate thopter tokens that can be used (similarly to Prossh, Skyraider of Kher) for various combos, including infinite damage, infinite mana, and infinite life
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The variety also means niche strategies always exist for creative brewers
Similarly, phasing in won't cause any "enters the battlefield" abilities to trigger