Superior Weapon Master

If you reduce a creature to 0hp using your GWM bonus attack ability you can them move up to 10ft and make another attack on another creature. Of you use the -5 to hit on this attack you add 10dmg on top of the 10 for GWM for a total of +20. If this reduces the creature to 0hp you can repeat the process and move 10ft to another creature and add +30dmg when using GWM's -5. You can continue doing this until you miss or fail to reduce a creature to 0hp.