obligatory (adj.)

• absolute
• binding
• compulsory
• conclusive
• decisive
• dictated
• essential
• exigent
• final
• hard-and-fast
• imperative
• imperious
• importunate
• imposed
• incumbent
• indispensable
• inevitable
• involuntary
• irrevocable
• mandatory
• must
• necessary
• necessitous
• peremptory
• prescriptive
• required
• requisite
• ultimate
• urgent