Complex sentence: He has yet to find one to fill up this fifth slot, however, it is implied that he intends to use a different alchemist to open the gate beforehand (such as Marcoh or Mustang).
Split into simpler sentences: He has yet to find one to fill up this fifth slot. However, it is implied that he intends to use a different alchemist to open the gate beforehand (such as Marcoh or Mustang).