The Collaborative International Dictionary of English v.0.48:

Decretory \Dec"re*to*ry\, a. [L. decretorius, from decretum. See
   Decree.]
   1. Established by a decree; definitive; settled.
      [1913 Webster]

            The decretory rigors of a condemning sentence.
                                                  --South.
      [1913 Webster]

   2. Serving to determine; critical. "The critical or decretory
      days." --Sir T. Browne.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "decretory":
   absolute, binding, commanding, compelling, compulsory, conclusive,
   decisive, decretal, decretive, dictated, dictating, directive,
   entailed, final, hard-and-fast, imperative, imposed, instructive,
   irrevocable, jussive, mandated, mandating, mandatory, must,
   obligating, obligatory, peremptory, preceptive, prescript,
   prescriptive, required, ultimate, without appeal