They were dark masters without doubt or ones who controlled them behind the curtains. He didn't care who they were, just wanted to know their identities to start planning a decisive blow for them in the future.
After the end of this war, he released his friends and team to collect more loot. The masters of this kingdom went towards their king, leaving behind endless corpses of monsters and dark masters. And William collected all of this loot for himself.
Weirdly enough, the masters who came with him from the other world showed great interest in doing the same. William didn't know that his effect on them was deep, and they learnt from his friends and team how to properly scavenge the battlefield.
As everything became calmer, William found a safe spot, isolated from all the ruckus going on around. It was a small hill, and there he carved a cave using his flying weapons, sat down, and started to examine the rings.