Fateful Duel

When Fateful Duel enters the battlefield choose a creature you control and a creature your opponent controls.
Both creatures cannot attack or block. At the start of each upkeep both creatures receive a -1/-1 counter. When either creature's toughness becomes 0, the other creature removes all -1/-1 counters and gets +1/+1 counters equal to the number of counters on the opposing creature before it died then sacrifice this card.