Knowledge Vault

Artifact    (CMC:4)

, : Exile the top card of your library face down. : Sacrifice Knowledge Vault. If you do, discard your hand, then put all cards exiled with Knowledge Vault into their owner's hand. When Knowledge Vault leaves the battlefield, put all cards exiled with Knowledge Vault into their owner's graveyard.

198 ME3 • ENAnthony S. Waters

™ & © 1993—2009 Wizards of the Coast

Legal in: Legacy,Vintage,Commander

Oracle Text (click to copy):

10/1/2008
You can activate the second ability any time you have priority, including in response to itself. However, you will only be able to sacrifice the Vault as the first such ability resolves.
10/1/2009
You don't get to look at the cards exiled with Knowledge Vault.
10/1/2009
If you activate Knowledge Vault's second ability, all that happens at that time is that you pay a cost of . As the ability resolves, you must sacrifice Knowledge Vault. This is mandatory. If you are able to do so, you discard your hand and put all cards exiled with Knowledge Vault into their owners' hands. If you aren't able to do so because Knowledge Vault left the battlefield or changed controllers before the ability resolves, nothing happens.