I think the best way would be to add a dictionary override for the calculated value on the "description" field on the task table, so that it has the same value as your custom field.
This way the "description" field (not HTML, just string text) will always have the value (contents) of what your users input to the "u_description" (custom HTML on rm_story) for that table (rm_story), but it won't affect the base table or any other tables that extend task.
Go to the base Task table and find the "description" field (column), open that and click on the "Advanced" UI Link (at the bottom, near the related lists)...and then go to "dictionary overrides" and click "New". Then, on the new Dictionary Entry Override form, select to override the calculated value for the field on the rm_story table & set the value to be "current.u_description" (or whatever you named the custom field).
Note that I could be wrong with the syntax...I think you want to use "current." but you may need to write it as a regular script function or maybe just "return current.u_field_name". Play around with it a bit here to fit your needs...but this should accomplish your goal, if I understand what you're looking for.
Also note that you can go "the other way", too - where you can set up a calculated value (no dictionary override needed) on the custom field so that it is updated with data inputted to the description field, if that extended field is available on the rm_story form...but I don't think that's what your looking for here as it won't leverage the HTML properties of the custom field in any way (defeats the purpose).
Further, know that you can't go both ways at the same time (you cant have both fields being calculated values of each other) - if you want the fields to be synchronous (so that either can be updated and both have same value/content, on the rm_story table), you'll have to write a Business Rule instead.
