The word "decretory" is quite rare and can be difficult to find many synonyms for. One option may be to use the more commonly used word "decree" instead. However, if you are looking for synonyms specific to "decretory", some options include "authoritative", "mandating", or "determinative". These words imply a sense of power or force behind a decision, which is similar to the connotations of "decretory". Other options may include "legislative", "prescriptive", or "mandatory". While these words may not be exact synonyms, they do evoke a similar sense of authority or power in making a decision.