As quoted on the official extension page:
If no one has earned a badge, it is naturally not common and will appear as legendary. There is also a comprehensive table below explaining how rarity levels are determined. The first “N” column represents the total number of members on the forum. The rarity of a badge is calculated by comparing the number of users who have that badge against the total member count.
In short, the more people who have a badge, the lower its rarity becomes automatically. If no one has it, it is considered legendary. There are a total of 5 rarity levels.
I'll continue to add more Badges as I work on the site and will always take suggestions you make in this thread!
