Demonic spirits: this translates “spirits of demons” (see the similar “the spirit of an unclean demon” in Luke 4.33). In translation it may not be advisable to say literally, as Good News Translation does, “the spirits of demons”; it will be better to say “demons” or “evil spirits,” or demonic spirits, as Revised Standard Version does.

Performing signs: for signs see 13.13-14.

Go abroad means “go out everywhere.” These spirits do not enter people and possess them, but they “go out to all the kings of the world, to bring them together for the battle” (Good News Translation). For the whole world the Greek phrase is “the whole inhabited earth” (as in 3.10). On the translation of kings see 1.5.

To assemble them for battle: the evil spirits bring together all the kings of the world for the battle of the End. For battle see 9.7. This phrase may also be rendered as “to bring together all the kings of the world to the place where they will fight (or, battlefield).”

The great day of God the Almighty: this is Judgment Day, when God will condemn and punish all evildoers. The battle will be between the forces of evil and the heavenly forces of God (see 19.11-15). For God the Almighty see similar language in 1.8; 4.8; 11.17; 16.7. This phrase may also be rendered as “The Day when God, who is all powerful, will judge people.”

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on The Revelation to John. (UBS Handbook Series). New York: UBS, 1993. For this and other handbooks for translators see here .