Eternity Clock

At the beginning of your end step, if you control
no planeswalkers, sacrifice Eternity Clock.
At the beginning of your end step, if you control
five or more planeswalkers with different
planeswalker types, exile Eternity Clock and take
an extra turn after this one.
It's chime signals the end. The end of the Guildpact,
of the Gatewatch, of Bolas' tyranny.