Granny 

Jung Sookja's grandson, Gwon Yonghee, is her entire world. But everything comes crashing down after he ends up in a coma. The culprits behind it, the infamous Black Star Mob, are set free despite all evidence. Then one day, two men suddenly appear in front of Mrs. Jung, telling her they will train her so she can enact revenge on her grandson's assailants and take down the mob once and for all. Hesitant at first, she decides to work with them and begins to hunt down her prey one by one. (Source: Tapas) Note: Includes six extra chapters.