“Slasher: Flesh & Blood” follows a wealthy, dysfunctional family—led by David Cronenberg as cruel patriarch Spencer Galloway—that gathers for a reunion on a secluded island only to learn they’ll be pitted against one another in a cruel game of life and death, all while being stalked by a mysterious masked killer. Nothing is what it seems and no one is safe as the tension—and body count—ratchets up.