The Millennium Calendar - 257 M LCI

Whenever you untap one or more permanents during your untap step, put that many time counters on The Millennium Calendar.
{2}, {T}: Double the number of time counters on The Millennium Calendar.
When there are 1,000 or more time counters on The Millennium Calendar, sacrifice it and each opponent loses 1,000 life.

mythic
Legendary Artifact
From the Magic: The Gathering set The Lost Caverns of Ixalan.