Colfenor's Urn 
Artifact
$0.28 #6993 0.21 18.9 years
Whenever a creature with toughness 4 or greater is put into your graveyard from the battlefield, you may exile it.
At the beginning of the end step, if three or more cards have been exiled with this artifact, sacrifice it. If you do, return those cards to the battlefield under their owner's control.
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Cards similar to Colfenor's Urn expected a card that isn't here?
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Purgatory86%
$1.54↑#28524↓0.45↑29.9 years↑
"Whenever a nontoken creature is put into your graveyard from the battlefield, exile that card."
+1 more matching line
Promise of Tomorrow86%
$0.31↑#16400↓0.43↑5.8 years↓
"At the beginning of each end step, if you control no creatures, sacrifice this enchantment and return all cards exiled with it to the battlefield under your control."
+1 more matching line
Chaos Shrine's Black Crystal78%
$4.18↑#12782↓0.00↓0.8 years↓
"Whenever a nontoken creature you control dies, exile it."
+1 more matching line
Angelic Renewal72%
$0.31↑#82950.33↑29.2 years↑
"Whenever a creature is put into your graveyard from the battlefield, you may sacrifice this enchantment. If you do, return that card to the battlefield."
Serpent's Soul-Jar72%
$0.31↑#16022↓0.62↑5.6 years↓
"Whenever an Elf you control dies, exile it."
Proper Burial70%
$3.59↑#17729↓0.36↑20.3 years↑
"Whenever a creature you control dies, you gain life equal to that creature's toughness."