I am looking for a comic webtoon, manhwa or manhua. For simplicity sake I am calling it a webtoon. It starts with the main character being burned and killed by a certain kingdom. They kidnapped her when she was a small child and she is reincarnated before she is taken. She is hiding under a bed and her mother finds her. Her mother dies through certain circumstances and the kingdom ends up kidnapping her again after her mom dies. After she's kidnapped, she is found in a dungeon by a guy who was spying on the kingdom and ends up saving her from the kingdom. She was tortured and abused because the kingdom wanted to steal her powers that she has that were similar to her mother's. She is reunited with her father and two siblings. She has blonde hair and her father and siblings have black hair.
Abducted from her mother at a young age and tortured for her incredible power, Evanna knew nothing but agony her whole life. That is until one day, she is rescued and returned to the father she never knew, the Duke of Vicente. But as Evanna’s very existence brings back painful memories for the Duke, he must learn how to love her, while Evanna must come to trust her father and heal her trauma. But with Evanna's power highly coveted, can these two overcome their thorny pasts for a rosier future?
The story opens with the MC being dragged out onto an altar, where she is sacrificed to demons (or something similar). The palisades around the area make it look a bit like she's going to be burned, but I don't think that's what's happening.
She then reincarnates to when she was a young child, still living with her mother. The scene with her mother finding her is just her trying to coax her from coming out from underneath the bed, where she is hiding in terror. Soldiers burst in and kidnap her again, presumably killing her mother.
She has her magical power drained again in the castle, and is discovered by someone in the dungeon. Presumably he is a spy, as mentioned in the question. She escapes with the help of a woman who introduces herself as a friend of her mother, and leads her to her father. He has black hair, as mentioned in the question.
Found by searching mangaupdates for the tags
Child Abuse and