Creatures you control cannot die. (If an effect, ability, or state-based action would move them from the battlefield to the graveyard, they remain on the battlefield instead.)
If a creature an opponent controls dies, return it to the battlefeild under your control as a blue snow Zombie in addition to it's other colors and types.
"We Free Folk have our stories, too. About how one of your King Crows found something... cold in the woods, with bright blue eyes. How he brought her home through your Wall and declared himself 'Night's King'." ― Ygritte