is it possible to verify Epic->some field contains value, when Task " is set to completed.
The task should not be able to be set to Completed, if the Epic->that field is not set.
Yes but most likely you need a scripted condition. There might be a many plugins that you can do this with but I would do it with http://www.adaptavist.com/w/products-plugins/adaptavist-scriptrunner/
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