This continuous expansion means that any definitive total count is merely a snapshot in time, quickly becoming outdated
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The three-mana cost is fine and the three life is nothing when you have a 40 life head start
Other prominent mechanics that incorporate (or freely enable) sacrifice include: Food, Clues, Treasures, Sagas, Unearth, Encore, Evoke, any other temporary tokens such as Faerie Artisans or Hostility, cards with Upkeep, Quests, Exploit, Casualty and now Bargain
The other nice thing about Atraxa is that she lets you run every color in Magic except red, giving you more choices when it comes to building your deck
(2/4/2023) Phasing out doesn't cause permanents to leave or re-enter the battlefield, so no "leaves the battlefield" or "enters the battlefield" abilities will trigger