A Study of Elritercept Alone or Together With Ruxolitinib in Adults With Myelofibrosis

Inclusion Criteria: 1. Ability to understand the purpose and risks of the study and provide signed and dated informed consent and authorization to use protected health information in accordance with national and local study participant privacy regulations. 2. In the opinion of the Investigator, the participant is able and willing to comply with the requirements of the protocol (e.g., all study procedures, return for follow-up visits). 3. Male or female greater than equal to (≥)18 years of age, at the time of signing informed consent. 4. Eastern Cooperative Oncology Group (ECOG) performance score lesser than equal to (≤)2. 5. Life expectancy ≥12 months per Investigator assessment. 6. Confirmed diagnosis of primary myelofibrosis (PMF) (prefibrotic or overtly fibrotic) according to the 2016 World Health Organization (WHO) criteria, post-polycythemia vera myelofibrosis (PV MF), or post-essential thrombocythemia myelofibrosis (ET MF) according to the 2008 International Working Group-Myeloproliferative Neoplasms Research and Treatment (IWG-MRT) criteria. 7. Anemia, defined as: 1. Having received ≥6 units of RBC transfusion for Hgb ≤8.5 g/dL in the 12 weeks prior to the planned C1D1, including ≥1 unit of RBC transfusion in the 28 days prior to C1D1; or 2. Having ≥3 evaluable Hgb measurements at less than (\<)10.0 g/dL including ≥1 evaluable Hgb measurement assessed 8 to 13 weeks prior to C1D1. Participants receiving RBC transfusions but not meeting criterion "a." may enroll under criterion "b." following the below parameters: * All pre-transfusion Hgb values (defined as a Hgb assessed within the 3 days prior to a transfusion) should be recorded, and ≥1 pre-transfusion Hgb value is required. * Hgb values collected within the 28 days following a transfusion will not be considered evaluable unless qualifying as a pre-transfusion Hgb; in cases where multiple transfusions are given in succession due to poor Hgb response, only the first pre-transfusion Hgb will be considered evaluable. 8. Arm-specific criteria: Arms 1A and 2A: 1. Previously treated with JAK inhibitor(s) and, per the Investigator, discontinued due to one of the following reasons: * Relapsed disease following treatment with JAK inhibitor(s) * Refractory to treatment with JAK inhibitor(s) * Intolerance to treatment with JAK inhibitor(s) * Participant no longer met risk/benefit ratio to continue JAK inhibitor(s) OR * Participant with prognostic score of intermediate-1 or higher per Dynamic International Prognostic Scoring System (DIPSS) and is ineligible for JAK inhibitor(s) in the opinion of the Investigator 2. Participants previously treated with JAK inhibitor(s) must have discontinued JAK inhibitor therapy ≥8 weeks before C1D1 Arms 1B and 2B: 1. Has been receiving ruxolitinib prescribed for a diagnosis of PMF (prefibrotic or overtly fibrotic), post-PV MF, or post-ET MF for ≥8 weeks prior to C1D1 and on a stable dose for ≥4 weeks prior to C1D1. In Arm 2B only, at least 10 participants should have been on ruxolitinib for \<6 months prior to C1D1. 2. Meets ≥1 of the following criteria in the opinion of the Investigator: * Current ruxolitinib treatment is considered to be providing insufficient control of the disease * The participant's cytopenias are limiting the participant's ruxolitinib dose intensity * The participant's disease is symptomatic and warrants additional therapy Arm 2C (Brazil only): 1. No prior treatment with JAK inhibitor(s) and no access to JAK inhibitor therapy as determined by the Investigator 2. Spleen volume ≥ 450 cubic centimeter (cm\^3) as assessed by CT or MRI collected during the pretreatment period and/or 3. Myelofibrosis Symptom Assessment Form Total Symptom Score (MF-SAF-TSS) meeting at least one of the following criteria during the pretreatment period: * 2 symptoms with average score ≥ 3 * Average total symptom score ≥ 10 9. Females of childbearing potential and sexually active males must agree to use highly effective methods of contraception as described in the protocol. Exclusion Criteria: Medical History: 1. Active infection requiring parenteral antibiotic therapy within 28 days prior to C1D1 or oral antibiotics within 14 days of C1D1. Prophylactic antibiotics and/or antifungals for neutropenia are allowed. 2. Presence of the following cardiac conditions: 1. New York Heart Association Class 3 or 4 heart failure 2. QTcF (QT interval corrected by Fridericia's formula) \>500 milliseconds (msec) on the screening or C1D1 electrocardiogram (ECG; mean of 3 measurements) 3. Uncontrolled clinically significant arrhythmia (participants with rate-controlled atrial fibrillation are not excluded) 4. Acute myocardial infarction or unstable angina pectoris ≤6 months prior to C1D1 3. Body mass index (BMI) ≥40 kilograms per meter square (kg/m\^2). 4. Presence of uncontrolled hypertension, defined as systolic blood pressure ≥160 millimeters of mercury (mmHg) or diastolic blood pressure ≥100 mmHg despite adequate treatment. 5. History of drug or alcohol abuse (as defined by the Investigator) within the past 2 years. 6. History of stroke, deep venous thrombosis, or arterial embolism within 6 months prior to C1D1. 7. Major surgery within 28 days prior to C1D1. Participants must have completely recovered from any previous surgery prior to C1D1 in the opinion of the Investigator. 8. Known positive for human immunodeficiency virus (HIV), active infectious hepatitis B with positive viral load (hepatitis B virus \[HBV\] deoxyribonucleic acid \[DNA\]), or active infectious hepatitis C with positive viral load (hepatitis C virus \[HCV\] ribonucleic acid \[RNA\]). Participants without a known positive history of HIV, HBV, and/or HCV do not require further testing, unless testing is mandated per local guidelines. 9. Any malignancy other than PMF, post-ET MF, or post-PV MF that has not been in remission a