What is the ruling of Islamic law regarding dealing with a grandmother who has severely harmed her grandchild, and is it permissible to curse her or seek revenge on her?

Your grandmother is a bad woman, and her giving you water of sorcery to wash with is an evil act. Hating her actions is not considered disobedience (to parents) as long as it is not accompanied by mistreating her. Honoring grandparents is obligatory, as they are in the same position as parents. If she commands you to abandon an evil, you may forbid her once, but if she does not respond, then keep silent and supplicate for her. You must be patient with her and pray for her guidance. It is not permissible to curse her or think of taking revenge on her, because a parent's right to dutifulness and kindness is not forfeited by their wrongdoing. Fear Allah and be patient with her harm, for Allah does not waste the reward of those who do good. You should occupy yourself with what benefits you, attend scholarly gatherings, keep company with the righteous, cooperate with them in righteousness and piety, and invite to goodness and reform.