A Legacy Begins...

When combat damage would be dealt to a blocking creature or player, roll a d20.
1-3: prevent damage dealt
4-17 damage is dealt normally
18-20 double damage dealt

If an attacking creature would destroy a creature, attacking creature gets an XP counter. Players may remove 3 XP counters from a creature they control and flip a coin. If heads, put 2 +1/+1 counters on it. If tails, put a +1/+1 counter on it.