Crohn's disease (CD) is a long-lasting condition causing inflammation that can affect any part of the gut. CD may cause tiredness, loose stools with or without bleeding, abdominal pain, weight loss, and fever. This study evaluates how safe and effective ABBV-154 is in participants treated for moderately to severely active CD. Adverse events and change in the disease activity will be assessed. ABBV-154 is an investigational drug being evaluated for the treatment of CD. In the induction period, there is a 1 in 5 chance that participants will be assigned to placebo. Depending on the dose received in the induction period, there is a 1 in 2 or 1 in 3 chance that participants will be assigned to placebo in the maintenance period. Around 265 participants 18-75 years of age with moderately to severely active CD will be enrolled in the study at approximately 200 sites worldwide. The study is comprised of a 12-week double-blind, placebo-controlled induction period, followed by either a 12-week double-blind re-induction period for non-responders or a 40-week double-blind placebo-controlled maintenance period for responders. In the maintenance period, responders will be randomized to receive subcutaneous placebo or ABBV-154 in 2 different doses every other week. Participants in the placebo group who are initial responders will receive ABBV-154 in the maintenance period. There may be higher treatment burden for participants in this trial compared to their standard of care due to study procedures. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.