Complex sentence: Geomancers are a lineage of men and women who, since 3500 BC, have been chosen by the Earth itself to defend it, with only one Geomancer operating at the time.
Split into simpler sentences: Geomancers are an order of men and women who, since 3500 BC, that have been chosen by the Earth itself to defend it. However, there is only one Geomancer operating at the time.