How does the mundane infusion work?

Mundane gives massive scaling depending on your lowest stat. If your lowest stat is at 20, Mundane gives physical scaling at 170, which is amazing.

For example, if you have 20s in your stats across the board except for INT, which you left at 5, Mundane will scale off INT and have +80 physical scaling. As you increase INT, the damage scaling will increase. Once INT hits 21, it's no longer the lowest stat as the others are at 20. At this point, Mundane will stay scaled to 170 since your lowest stat is at 20.

According to the Dark Souls 2 wikidot, the exact mundane scaling goes as follows: