We killed the self prophetic "Kell of Kells". Logically, it makes sense that the House of Kings would use the failed work of Skolas, to band the broken Houses into Order and underneath their banner to assault the City once more. Think about it, most of the City has it's Guardians spread across the system fighting the Taken. It's a small opportunity that the House of Kings would get. The Kell of House Kings has been the only Kell to evade the City for so long. It's only a matter of time before he re-appears.