This can be stated in active form. Alternate translation: "The rider of the horse killed the remainder of the beast's armies with the sword that extended from his mouth" (See: [[rc://en/ta/man/translate/figs-activepassive]])
The sword blade was sticking out of his mouth. The sword itself was not in motion. See how you translated a similar phrase in [Revelation 1:16](../01/16.md).