Karn, Scion of Urza Hour of Devastation then sacrifice it unless you

then sacrifice it unless you pay its upkeep cost for each age counter on it

−2: Look at the top four cards of your library

−3: Reveal the top four cards of your library

They also serve as a great way to store a sideboard or cards together

Karn, Scion of Urza Hour of Devastation then sacrifice it unless you+1: Reveal the top two cards of your library. An opponent chooses one of them. Put that card into your hand and exile the other with a silver counter on it. 1: Put a card you own with a silver counter on it from exile into your hand. 2: Create a 0 0 colorless Construct artifact creature token with "This token gets +1 +1 for each artifact you control."