Reign 4×16 “All It Cost Her” Season 4 Episode 16 Extended Promo (Series Finale) – SERIES FINALE – Mary (Adelaide Kane) makes a decision that will change the course of history — forever shaping the fate of her nation, her crown, and her newborn son. Mary’s perilous move for her survival causes the stunning realization that perhaps she is not the only one who may not survive. Meanwhile, Elizabeth (Rachel Skarsten) steadies herself for war and will become the instigator in Mary’s demise while later realizing she and Mary might be more similar than either thought. Dan Jeannotte, Jonathan Goad, Celina Sinden, Megan Follows, Craig Parker, Ben Geurens, Rose Williams, Will Kemp and Spencer MacPherson star. Holly Dale directed the episode written by April Blair and Laurie McCarthy (#416). Original airdate 6/19/2017.