It wasn't immediately clear to Thorin what made Riona's power so unique. Though he had his suspicions about her significant strength, the reality proved far more bizarre than he had anticipated.
Vampires were strong—he admitted that, but Riona's level was beyond comprehension. The fact that she was the Blood Moon might explain her strength.
However, it didn't explain why she couldn't transform into a bat and why she didn't grow fangs.
It bothered Thorin, but he didn't think too much of it. At that time, Thorin was only focusing on how to get the Blood Moon child to give him the power he deserved to have.
Initially, Thorin had planned to gain the Blood Moon child's trust through bribery. He had intended to slowly get close to whoever it was, and once he gained the Blood Moon child's trust, he would ask directly.