Riven, Taken Ahmkara

Riven costs {1} less to cast for each permanent you control taken from an opponent.

When you cast this spell Take half the cards in each opponents hand rounded up. Place all permanents taken this way onto the battlefield under your control. You may cast instant or sorcery spells taken this way without paying their mana cost as long as you control Riven.

Flying, Protection from Activated and Triggered Abilities.