Wyxilla, the Sudden Chill

Flash
Your opponents can't cast spells
during combat.
Whenever you cast your first noninstant spell
during each opponent's turn, that player gets an oblivious counter.
At the beginning of each opponent's upkeep,
untap target permanent you control for each oblivious counter that player has.
"We've been here since before you were born. Pity
you're only now seeing us."