A Study to Investigate How Well Ravagalimab (ABBV-323) Works and How Safe it is in Participants With Moderate to Severe Ulcerative Colitis Who Failed Prior Therapy

* Participants must voluntarily sign and date an informed consent, approved by an independent ethics committee (IEC)/institutional review board (IRB), prior to the initiation of any screening or study-specific procedures. * Diagnosis of UC for at least 3 months prior to Baseline. Appropriate documentation of biopsy results consistent with the diagnosis of UC in the assessment of the Investigator, must be available. * Participant meets the following disease activity criteria: Active UC with an Adapted Mayo score of 5 to 9 points and endoscopic subscore of 2 to 3 (confirmed by central review). * History of inadequate response, loss of response, or intolerance to one or more of the approved biologic therapies: infliximab, adalimumab, golimumab, vedolizumab, and/or tofacitinib (Note: If tofacitinib was received in a clinical trial, subject must have received open-label drug).

* Participant having an active, chronic, or recurrent infection that based on Investigator's clinical assessment makes the participant an unsuitable candidate for the study. * Participant having any malignancy except for successfully treated non-metastatic cutaneous squamous cell or basal cell carcinoma or localized carcinoma in situ of the cervix. * Participant with history of dysplasia of the gastrointestinal tract or evidence of dysplasia in any biopsy performed during the screening endoscopy other than completely removed low-grade dysplastic lesions. * Laboratory values not meeting the following criteria : Serum aspartate transaminase (AST) and alanine transaminase (ALT) \<= 2\* upper limit of normal (ULN); Total white blood cell (WBC) count \>= 3.0\*10\^9/L.