Complex sentence: It is only logical for the Council to keep close watch on the wizards, being their only real challenge for power, as Archmage Gromph has many responsibilities in the City of Spiders.
Split into simpler sentences: It is only logical for the Council to keep close watch on the wizards, being there only real challenge for power. And as Archmage Gromph has many responsibilities in the City of Spiders.