Dominaria Spoiler Digest - Who's Who and What's What from the Release Notes
Soulbond (You may pair this creature with another unpaired creature when either enters the battlefield. They remain paired for as long as you control both of them.)
As long as Druid's Familiar is paired with another creature, each of those creatures gets +2/+2.
5/1/2012 If Druid’s Familiar becomes unpaired, it will immediately lose the +2/+2 bonus. If this causes it to have damage marked on it equal to it or greater than its new toughness, it will be destroyed. The same is true for the creature it was paired with.