Valerian scared Telia into submitting, and then he recreated her arms. The threat of turning her into a mound of ash was untrue. He had implanted a wisp of decay mana in her new body. Suppose she was to turn against him. The mana will eat her up from the inside out.
Telia's arms were recreated right before the eyes of the Knights and the Enemies. They were shocked because even commoners knew that things did not work like this and that magic was not all-powerful. This spell and magic bring you glory, treasures, and recognition.
However, it also brings envy, jealousy, betrayal, and loneliness. Valerian was not bothered about all these things. His only goal was not to simply kill Immortuous, but he wanted to destroy him. Everything that the Tyrant had, allies, loyal subordinates, and family. All of it, Valerian wanted to destroy.