When the lamb is down and torn asunder, it awakens the GOAT from their slumber. In this case, the Lamb was likely pretty badly injured during a crusade, which allowed a puddle of dark liquid to appear. But to wake? All it took was one drop of the lamb's blood to unlock the darker version of itself. Its mirror. And who knew what would happen past that.

It could do the same rituals as the lamb, even had the same theme, but different coloring, as would soon be seen, if the lamb dared gaze.