The Last Great Magician Ol

The male protagonist traveled through time and obtained the system of killing enemies and becoming stronger. Xiao Yao encountered fantasy creatures for the first time at the age of 14 and realized their terror, so he began a life of learning to fight monsters during the day and at night until he killed the Fantasy King at the age of 22... After the death of the Fantasy King, the fantasy creatures fell into a slump, and the probability of their appearance dropped again and again until they disappeared. This made Xiaoyao mistakenly think that his battle was completely over. It was not until a year later that he discovered one night that his sister had turned into a magical girl, and fought with the long-lost fantasy creatures again... Xiao Yao:? ? How can my sister be a magical girl?