Red aggro decks were dominant thanks to the existence of Embercleave
This is the creature that enables infinite loops with Ghostly Flicker and the reason blink decks can combo off
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You need to sacrifice a Merfolk to do this, and it only provides a Force Spike, but turning all of your Merfolks into Force Spikes can help stop a crucial spell at the last moment
For Aristocrats or sacrifice-themed decks, its Harness ability can be used as an instant-speed sacrifice outlet, while its reanimation brings back a creature you sacrificed for value
Elite Spellbinder 3/1 flying is already a fast clock, but the most annoying part of dealing with Elite Spellbinder is that it makes you pay more on a given card, so its a nice disruptive element for aggro decks