Death's Oasis

Whenever a nontoken creature you control dies, mill two cards. Then return a creature card with lesser mana value than the creature that died from your graveyard to your hand.
{1}, Sacrifice this enchantment: You gain life equal to the highest mana value among creatures you control.

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Because the triggered ability doesn't target the creature card in your graveyard, you may choose one of the two cards you put there from your library.

If Death's Oasis becomes a creature, activating its last ability will cause its first ability to trigger.

If a creature on the battlefield or a card in your graveyard has {X} in its mana cost, X is considered to be 0.

Use the mana value of the creature as it last existed on the battlefield and the mana value of the creature card as it exists in your graveyard to determine which cards may be returned to your hand. This may result in a creature's death bringing itself back to your hand; perhaps it was one card of a merged creature, or it had become a copy of a more expensive creature.