Pure Soul

Sunny reasoned that the Rat King could not be merely a swarm of mindless vermin — he had battled hordes of abominations before, and they were not at all like the enemy he faced today. The entity he was facing was indeed a swarm, but one guided by a single Will. 

Which hinted that there was a single source of that Will — the origin of the innumerable legion of abominable rats that lusted to devour Slayer in frenzied hunger. Even if there had not been, the Snow Tyrant would have forced it upon the Rat King…

After all, it needed to control the Snow Beast figure somehow, and Sunny doubted that his adversary could freely create an endless number of invisible strings to subjugate each individual rat.