I am trying to build a class feature (in aubclass) that uses the bardic inspiration die to enhance a melee attack and expends one use of bardic inspiration. Here is the text for this feature:

Smiting Song

At 6th level, you gain a powerful smiting song that harms or even incapacitates an opponent. When you hit another creature with a melee weapon attack, you can expend one use of Bardic Inspiration to enhance the attack with thunder damage. You add your Bardic Inspiration die to the attack’s damage roll (as thunder damage), and the target must succeed on a Constitution saving throw against your spell save DC or be stunned until the end of your next turn.

Does anyone have suggestions on how to build this in homebrew? I am racking my brain on this and I can't seem to find the right way to do this mechanically. Using Actions? Using modifiers? Linking it back to bardic inspriation?

There is no way to link it back, you would need to create an action that would appear to require a Save Vs. Damage setup, but simply use the text to explain it is automatic Damage and Save Vs. Condition instead. Then you would edit the action to use higher level scaling for the action to increase the die size at the appropriate levels to match Bardic Inspiration.

Thank you very much!