Riven the Insidious

{u}{b}, {t} Put a Contract counter on Riven the Insidious. Contract counters remain on Riven as it
moves to any zone other than a player's hand or library.
At the beginning of your end step you may choose
an opponent. At the beginning of that player's next
upkeep they sacrifice a nonland permanent for each Contract counter on Riven. For each permanent sacrificed this way, put a +0/+1 counter on Riven.