Nim Deathmantle triggers, causing you to pay ({4} magic symbol) to return
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Ashaya from your graveyard to the battlefield, and attaching
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Nim Deathmantle to it.
When
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Ashaya enters the battlefield,
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The Necrobloom triggers, creating a 0/1 Plant or a 2/2 Zombie creature token.
Activate
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Ashnod's Altar by sacrificing the creature token, adding ({C} magic symbol) ({C} magic symbol) .
Repeat.
If you control a nontoken creature that has haste, you may instead sacrifice it, and then tap it each iteration to get infinite green mana and colorless mana.