Complex sentence: He also returned Honorius' sister Galla Placidia to him, and in return Wallia was granted Aquitaine in 417 as a region where the Visigoths would be based as "foederati".
Split into simpler sentences: He also returned Honorius' sister Galla Placidia to him. As gains from these arrangements, Wallia was granted Aquitaine in 417 as a region where the Visigoths would be based as official allies or "foederati".