Eirlys, Frost Sorceress

Prowess, frosttouch (Whenever this creature deals
damage to a creature, tap that creature. It doesn't
untap during its controller's next untap step.)
Whenever you cast a blue spell, you may pay {u}. If you do, untap Eirlys, Frost Sorceress.
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When the bite of winter rolls in, you should know that
she is not far behind.