Inclusion Criteria:

          -  Completion of informed consent prior to any study specific procedures

          -  Patients must agree to tissue collection for correlative studies at the specified
             timepoints

          -  Male aged 18 years and above

          -  Histologically or cytologically confirmed prostate carcinoma

          -  Localized or regional high-risk disease as defined by at least one of the following
             features: Prostate specific antigen (PSA) > 20 ng/mL, T3a or higher, grade group 4-5
             (i.e. Gleason score > 8) as per National Comprehensive Cancer Network (NCCN) Prostate
             Cancer Version 2.2020 for high risk or very high risk prostate cancer, and/or regional
             lymph nodes positive for prostate cancer

          -  Planned for definitive treatment of local regional prostate cancer using XRT and
             androgen ablation

          -  Willing to undergo ongoing medical castration to maintain testosterone levels of =< 50
             ng/dL (=< 2.0 nM) throughout systemic treatment or have undergone bilateral
             orchiectomy

          -  Eastern Cooperative Oncology Group (ECOG) performance status of =< 2

          -  Hemoglobin >= 10.0 g/dL (measured within 7 days prior to treatment registration)

          -  Absolute neutrophil count (ANC) >= 1.5 x 10^9/L (measured within 7 days prior to
             treatment registration)

          -  White blood cells (WBC) > 3 x 10^9/L (measured within 7 days prior to treatment
             registration)

          -  No features suggestive of myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML)
             on peripheral blood smear

          -  Platelet count >= 100 x 10^9/L (measured within 7 days prior to treatment
             registration)

          -  Total bilirubin =< 1.5 x institutional upper limit of normal (ULN) (except for
             patients with known Gilbert's disease) (measured within 7 days prior to treatment
             registration). (Note: In subjects with Gilbert's syndrome, if total bilirubin is > 1.5
             x ULN, measure direct and indirect bilirubin and if direct bilirubin is =< 1.5 x ULN,
             subject may be eligible.)

          -  Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase [SGOT]) and
             alanine aminotransferase (ALT) (serum glutamic pyruvic transaminase [SGPT]) =< 2.5 x
             institutional upper limit of normal (measured within 7 days prior to treatment
             registration)

          -  Calculated creatinine clearance (Cockcroft-Gault Equation) >= 30 mL/min (measured
             within 7 days prior to treatment registration)

          -  Serum Albumin > 3.0 (measured within 7 days prior to treatment registration)

          -  Serum potassium >= 3.5 mmol/L (measured within 7 days prior to treatment registration)

          -  Able to swallow study drugs whole as a tablet/capsule

          -  Patients who have partners of childbearing potential (e.g. female that has not been
             surgically sterilized or who are not amenorrheic for >= 12 months) must be willing to
             use two methods of birth control including adequate barrier protection during the
             study and for 3 months after last dose of niraparib, abiraterone acetate, and/or
             apalutamide administration. In addition men should not donate sperm during this
             period. Please note that the efficacy of hormonal contraception may be decreased if
             administered with niraparib, abiraterone acetate, and/or apalutamide

          -  Patient is willing and able to comply with the protocol for the duration of the study
             including undergoing treatment and scheduled visits and examinations including follow
             up

          -  Medications known to lower the seizure threshold must be discontinued or substituted
             at least 4 weeks prior to study entry

        Exclusion Criteria:

          -  Any prior systemic treatment for prostate cancer with the exception of ADT started
             within 3 months of trial enrollment. Any prior PARP inhibitor therapy

          -  Patients who have prostate cancer with distant metastatic disease

          -  Patients who have had prior major surgery (prostatectomy) or radiotherapy for the
             treatment of prostate cancer

          -  Any unresolved toxicity (Common Terminology Criteria for Adverse Events [CTCAE] grade
             >= 2) from previous anti-cancer therapies

          -  History or current diagnosis of MDS/AML, and/or history of any malignancy [other than
             the one treated in this study] which has a >= 30% probability of recurrence within 24
             months (except for adequately treated non-melanoma skin cancer, curatively treated
             in-situ cancer of the cervix or Ta urothelial carcinomas)

          -  Active uncontrolled infection (patients completing a course of antibiotic or antiviral
             therapy whose infection is deemed to be controlled may be allowed on study after
             discussion with the principal investigator [PI]; the PI will serve as the final
             arbiter regarding eligibility)

          -  Active or symptomatic viral hepatitis or chronic liver disease

          -  Active pneumonitis or extensive bilateral lung disease of non-malignant etiology

          -  Any underlying medical or psychiatric condition, which in the opinion of the
             Investigator, will make the administration of study drug hazardous or obscure the
             interpretation of adverse events. Examples include, but are not limited to superior
             vena cava syndrome, extensive bilateral lung disease on high resolution computed
             tomography (HRCT) scan, uncontrolled seizures, history of allogeneic organ transplant,
             history of primary immunodeficiency or any psychiatric disorder that prohibits
             obtaining informed consent

          -  Patients unable to swallow orally administered medication and patients with
             gastrointestinal disorders likely to interfere with absorption of study medication

          -  Patients with a known hypersensitivity to niraparib, apalutamide, and/or abiraterone
             acetate

          -  Prisoners or subjects who are involuntarily incarcerated

          -  Subjects who are compulsorily detained for treatment of either a psychiatric or
             physical (e.g. infectious disease) illness

          -  Seizure or known condition that may pre-dispose to seizure (e.g. prior stroke within
             1year to randomization, brain arteriovenous malformation, Schwannoma, meningioma, or
             other benign central nervous system [CNS] or meningeal disease which may require
             treatment with surgery or radiation therapy)

          -  Severe or unstable angina, myocardial infarction (within 6 months prior to
             enrollment), symptomatic congestive heart failure, arterial or venous thromboembolic
             events (e.g., pulmonary embolism, cerebrovascular accident including transient
             ischemic attacks), uncontrolled hypertension, or clinically significant ventricular
             arrhythmias within 6 months prior to randomization

          -  Current evidence of any of the following:

               -  Gastrointestinal disorder affecting absorption

               -  Active uncontrolled infection (e.g., human immunodeficiency virus [HIV] or viral
                  hepatitis)

               -  Any chronic medical condition requiring a higher dose of corticosteroid than 10
                  mg prednisone/prednisolone once daily

               -  Avoid concomitant strong CYP3A4 inducers during abiraterone acetate treatment. If
                  a strong CYP3A4 inducer must be co-administered, increase the abiraterone acetate
                  dosing frequency

               -  Avoid co-administration of abiraterone acetate with CYP2D6 substrates that have a
                  narrow therapeutic index. If an alternative treatment cannot be used, exercise
                  caution and consider a dose reduction of the concomitant CYP2D6 substrate

               -  Baseline moderate and severe hepatic impairment (Child-Pugh class B & C)

               -  Any condition that in the opinion of the investigator, would preclude
                  participation in this study