They gathered (sunēgagen). Second aorist active indicative of sunagō, singular (the three unclean spirits), like ekporeuetai in Rev 16:14.
Har-Magedon (HaṙMagedōn). John proceeds now after the interruption in Rev 16:15. Perhaps “the mountains of Megiddo” though not certain. Megiddo is in the valley of Esdraelon, and by the waters of Megiddo (the Kishon) Israel gained a decisive victory over Sisera (Jdg 5:19), celebrated in Deborah’s song. See also Rev 20:8. and Eze 39:2, Eze 39:4.