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."
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.
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.