The general idea is that athels younger sister (still thimkin name) was very talented since young and got chosen to train as first, but her connection to the fade n demons was too strong, and when templars found the clan, she became an abomination +

Which resulted in her death and the mothers death (trying to stop the daughter), living old kind grumpy dad + athel Athel saw it all happen, tere was fire involved, he became fixated with the idea of correcting wrongs and started to train as first, even without calling, and he tries his best :')