Because damage remains marked on a creature until it's removed as the turn ends, lethal damage dealt to other creatures you control with flying may cause them to be destroyed if Sephara leaves the battlefield during that turn.

To pay Sephara's alternative cost, you may tap any untapped creatures you control with flying, including ones you haven't controlled continuously since the beginning of your most recent turn.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as Sephara's alternative cost), add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.