Norrix placed the rabbits he had caught in cages and whistled for leprechauns to come pick them up. Rats, rabbits, heck, even snails. Atha wanted to experiment with small things. But, Norrix had his eyes on something bigger.
There was a giant in the forest. One that Norrix had under compulsion and that was making his way to them. Atha might be squeamish about using his soul, but a soul like that could protect Alcandino from just about everything, really.
Norrix heard the thundering steps, and he went to check up on the giant. The vampire had no idea if the humanoid had it coming. He did not try to tell himself that most giants were human eaters. But he knew that, the greater the soul, the greater the barrier.
That was the principle that was behind SSS ranked barrier stones, and, for the O ranked barrier stones that Atha wanted to produce, the principle should hold true.