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Gems Gems are the pay-to-play currency in MTG Arena

Most of those, like Ertai Resurrected, have some sort of likely downside

If The Millenium Calendar's last ability is countered or otherwise removed from the stack, it will just trigger again immediately if The Millenium Calendar still has 1,000 or more time counters on it

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It also works well with commanders that have powerful Enters the Battlefield effects you want to repeat

Note that Fenrir counts itself for the Ecliptic Growl trigger, so your opponent needs to play a 4/x to stop it from drawing