Youjo Musou: Nakama Ni Uragirareta Shoukanshi, Mazoku No Oujo Ni Natte "eirei Shoukan" De Dekiai Slow Life Wo Okuru summary:

Lilith, the summoner who accompanied the hero party, was betrayed during the battle with the demons and suffered a serious injury because she refused the invitation of the hero. Helped by her demons, she cannot return to the country in a detour, so she decides to become a demon at the recommendation of Astaroth, the four demon kings who helped her. However, she made a slight mistake and she turned into a little girl. Even if she becomes a little girl, her magical power is not the same, but powers up! Summon the spirits and make an outstanding performance in battle and slow life!