The purpose of this study is to assess the feasibility, safety and efficacy of H3K27M-specific engineered immune effector (EIE) therapy in patients with high-risk, H3K27M-positive diffuse midline glioma/diffuse intrinsic pontine glioma. Another goal of the study is to learn more about the function of the anti-H3K27M EIE cells and their persistency in patients.