Complex sentence: Bennett tasked Mahaddie with finding and training up crews to replace the group's losses, a task at which he became notoriously successful, earning himself the moniker of "Don Bennett's Horse Thief".
Split into simpler sentences: Bennett tasked Mahaddie with finding and training up crews to replace the group's losses. This was a job Mahaddie became notoriously successful at, earning himself the moniker of "Don Bennett's Horse Thief".