Vermin Trainer( Drow of the Underdark, p. 53)
Your service to the Spider Queen gives you insight into training vermin.
You can use the Handle Animal skill to handle and train vermin as if they were animals with Intelligence scores of 1.
The size of vermin directly affects the ability of the trainer to instruct them (see page 46).
Without this feat, only animals and magical beasts with Intelligence scores of 1 or 2 can be handled and trained with the Handle Animal skill, and using Handle Animal on a magical beast is done with a —4 penalty on the check.