Wild magic hammer

I have no idea how to setup a Macro for an extremely complex weapon. Would a weapon that randomly selects a set of damage dice and effects from a table each time it rolls damage be possible? Best course of action would be something like: attack roll>damage roll>rolls from 1 to 20>result is the set of damage dice and effects> shows the result. I can get the result in a 5e Damage template, but i can't figure out the random selection. 

Can you supply more information on the mechanic? Like a step by step process if you were to roll it manually at the kitchen tab le?

Given 20 different damage templates, when activated the macro needs to roll a d20 and choose a corresponding damage template based on the roll. The chosen template is the output of the macro. If templates are a problem then text is fine, but i don't know if it could autoroll the damage as text.