Call the Everwinter

Synergize {7}{u}{u} (You may cast this spell for its synergize cost. If you do, exile this spell as it resolves,
then, you may cast another spell you own in exile with Synergize without paying its mana cost)
Tap any number of target creatures. Those
creatures don't untap during their controller's
next untap step.
If this spell's synergize cost was paid, copy the
spell you cast from exile.