Complex sentence: The Gatekeeper was able to summon the spirits of past monsters with his staff and control them, however he lost control when the Rangers destroyed his staff and the fallen monsters attempted to attack both him and Olympius.
Split into simpler sentences: The Gatekeeper was able to summon the spirits of past monsters with his staff and control them. However, he lost control when the Rangers destroyed his staff and the fallen monsters attempted to attack both him and Olympius.