Urza's Clock

Urza's Clock may only enter the battlefield when
you have five or more artifacts on the battlefield.
When another artifact enters the battlefield,
put a time counter on Urza's Clock.
Artifacts you control have "Pay 3 life: untap this artifact."
When Urza's Clock has 10 or more time
counters on it, you may fetch Urza's Tome
then remove all time counters from Urza's Clock.
Fetch: Search your library for the named card,
put it onto the battlefield, then shuffle your library.