Brood Keeper

Oracle Text

Whenever an Aura becomes attached to Brood Keeper, create a 2/2 red Dragon creature token with flying. It has "R: This creature gets +1/+0 until end of turn."

Card Rulings

7/18/2014 Brood Keeper’s ability triggers both whenever an Aura enters the battlefield attached to Brood Keeper and whenever an Aura on the battlefield attached to a different object becomes attached to Brood Keeper.
7/18/2014 If an Aura becomes attached to Brood Keeper that causes another player to gain control of it, that player will control the triggered ability that creates a Dragon.