Retifanlimab with Bevacizumab and Hypofractionated Radiotherapy for the Treatment of Recurrent Glioblastoma

Inclusion Criteria: * Age ≥ 18 years * Recurrent World Health Organization (WHO) grade IV glioblastoma. Note: Any number of recurrences are allowable. Glioblastoma (GBM) variants and molecular GBM are allowed * Candidates for radiotherapy * Prior use of bevacizumab is allowed as long as the last treatment is \> 4 months prior to randomization * Dexamethasone dose ≤ 4mg daily at the time of randomization (higher dose of steroid for symptom control is allowed during the study) * Karnofsky performance status ≥ 60% * Measurable disease or non-measurable disease per Response Assessment in Neuro-Oncology (RANO) criteria * Absolute neutrophil count (ANC) ≥ 1,500/mm\^3 (obtained ≤ 28 days prior to registration) * NOTE: If your site laboratory reports use different units of measurement than what is required by the protocol eligibility requirements, please use the "Lab Test Unit Conversion Worksheet" available on the Academic and Community Cancer Research Untied (ACCRU) website under "General Forms" * Platelet count ≥ 100,000/mm\^3 (obtained ≤ 28 days prior to registration) * NOTE: If your site laboratory reports use different units of measurement than what is required by the protocol eligibility requirements, please use the "Lab Test Unit Conversion Worksheet" available on the ACCRU website under "General Forms" * Hemoglobin ≥ 9.0 g/dL (obtained ≤ 28 days prior to registration) * NOTE: If your site laboratory reports use different units of measurement than what is required by the protocol eligibility requirements, please use the "Lab Test Unit Conversion Worksheet" available on the ACCRU website under "General Forms" * Total bilirubin ≤ 1.5 x upper limit of normal (ULN) (obtained ≤ 28 days prior to registration) * NOTE: If your site laboratory reports use different units of measurement than what is required by the protocol eligibility requirements, please use the "Lab Test Unit Conversion Worksheet" available on the ACCRU website under "General Forms" * Alanine aminotransferase (ALT) and aspartate transaminase (AST) ≤ 3 x ULN (≤ 5 x ULN for patients with liver involvement) (obtained ≤ 28 days prior to registration) * NOTE: If your site laboratory reports use different units of measurement than what is required by the protocol eligibility requirements, please use the "Lab Test Unit Conversion Worksheet" available on the ACCRU website under "General Forms" * Creatinine ≤ 1.5 x ULN (obtained ≤ 28 days prior to registration) * NOTE: If your site laboratory reports use different units of measurement than what is required by the protocol eligibility requirements, please use the "Lab Test Unit Conversion Worksheet" available on the ACCRU website under "General Forms" * Negative pregnancy test done ≤ 14 days prior to registration for women of childbearing potential only. (Pregnancy test can be urine or serum.) * NOTE: If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. * A female of childbearing potential is a sexually mature female who: * 1\) Has not undergone a hysterectomy or bilateral hysterectomy; or * 2\) Has not been naturally postmenopausal for at least 12 consecutive months (i.e. has had menses at any time in the preceding 12 consecutive months) * Provide informed written consent ≤ 28 days prior to registration * Willing to return to enrolling institution for follow-up (during the active monitoring phase of the study, i.e., active treatment) * Willing to provide mandatory blood specimens for correlative research purposes Exclusion Criteria: * Any of the following because this study involves an investigational agent whose genotoxic, mutagenic, and teratogenic effects on the developing fetus and newborn are unknown: * Pregnant persons * Nursing persons * Persons of childbearing potential who are unwilling to employ adequate contraception (men and women) * Co-morbid systemic illness or other severe concurrent disease which, in the judgement of the investigator, would make the patient inappropriate for entry into the study or interfere significantly with the proper assessment of safety and toxicity of the prescribed regimens * Receiving any other investigational agent which would be considered treatment for the primary neoplasm ≤ 2 weeks prior to registration * Active uncontrolled autoimmune disease or syndrome (i.e. moderate or severe rheumatoid arthritis, moderate or severe psoriasis, multiple sclerosis, active inflammatory bowel disease) that has required systemic treatment in the past 2 years (i.e. with use of disease-modifying agents, corticosteroids, or immunosuppressive drugs) or who are receiving systemic therapy for an autoimmune or inflammatory disease (i.e. with use of disease modifying agents, corticosteroids, or immunosuppressive drugs). Replacement therapy (e.g., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency) is not considered a form of systemic treatment. Subjects are permitted to enroll if they have vitiligo, resolved childhood asthma/atopy, type I diabetes mellitus, residual hypothyroidism due to autoimmune condition only requiring hormone replacement, psoriasis not requiring systemic treatment, or conditions not expected to recur in the absence of an external trigger * Has a severe acute or chronic medical condition including immune colitis, inflammatory bowel disease (may be enrolled at the discretion of the principal investigator \[PI\]), immune pneumonitis, or laboratory abnormalities that may increase the risk associated with study participation or study treatment administration or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the patient inappropriate for entry into this study * A history of allergic reactions attributed to compounds of similar chemical or biologic composition to epacadostat, retifanlimab, bevacizumab, or other agents used in the study * Has