March of Swirling Mist — Kamigawa: Neon Dynasty Promos #61p

As an additional cost to cast this spell, you may exile any number of blue cards from your hand. This spell costs less to cast for each card exiled this way.

Up to X target creatures phase out. (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.)