March of Swirling Mist

“As an additional cost to cast this spell, you may exile any number of blue cards from your hand. This spell costs 2 less to cast for each card exiled this way. <br>Up to X target creatures phase out. <em>(While they're phased out, they're treated as though they don't exist. Each one phases in before its controller untaps during their next untap step.)</em>”