Seven Brides, Seven Seals

At the beginning of your upkeep, put an Armageddon counter on Seven Brides, Seven Seals. Then, create a X/X black Demon token, where X is equal to the number of Armageddon counters on it. If Seven Brides, Seven Seals has seven or more Armageddon counters on it, exile it and return it to the battlefield transformed instead.