Chronomantic Hourglass

At the beginning of your upkeep, you may tap Chronomantic Hourglass.
If you do, exile the top card of your library
face down.
{3},{t}:Put a time counter on each face-down
exiled card you own.
{x},{t},Sacrifice Chronomantic Hourglass:
Take an extra turn after this one. You may cast spells
and activate abilities only from among the exiled cards with time counters, and you may spend mana as
though it were mana of any color to cast those spells.