Legion of Souls (3.5e Feat)
From D&D Wiki
 Legion of Souls [Binding]
Allows a Binder to merge the souls of two or more creatures of the same species.
Prerequisite: 18 ranks in Knowledge (Planes) or Knowledge (Arcana), and Soul Merging Class Feature
Benefit: You can now use Soul Merge with both the Bound Creature and Secondary target being of the same race, but this can only be done once per Secondary target.
Special: Can be taken by a Binder as a Bonus Feat.