Hi
I know my question is quit about everything, that is because I am a bit confused here.
I want to create a Condition that through an error message to the user rather than just not allow them to make the transition on the workflow. A Validator did not help as It did not allow checking values of custom fields. And I do not really know how the user should use it (when I add validation to change value before transition for a certain existing field I get an error message even after changing that value. So not sure how that really works).
From reading other questions/answers, it seems I need to create custom Condition/Validator. On this front, I have two questions:
1- Can I create such a thing on Cloud JIRA? Or it has to be JIRA Server instance?
2- Where do I start with that exactly? Do i need to first create a Plugin, which hosts a Module, which then contains my conditional logic? Or what?? And what is that OnDemand?
I really hope someone can put me in the right path as now I am lost 
Thanks
Fadi