Complex sentence: However, they failed to recognize the Catalyst as nothing more than a tool to fulfill its mandate, and eventually their creation betrayed them, deciding that the Leviathans also risked conflict with synthetics.
Split into simpler sentences: However, they failed to recognize the Catalyst as nothing more than a tool to fulfill its mandate, and eventually their creation betrayed them. It decided that the Leviathans also risked conflict with synthetics.