Jamieson, Fausset and Brown on Revelation 11:7

7. finished their testimony--The same verb is used of Paul's ending his ministry by a violent death.

the beast that ascended out of the bottomless pit--Greek, "the wild beast ... the abyss." This beast was not mentioned before, yet he is introduced as "the beast," because he had already been described by Daniel (Da 7:3, 11), and he is fully so in the subsequent part of the Apocalypse, namely, Re 13:1; 17:8. Thus, John at once appropriates the Old Testament prophecies; and also, viewing his whole subject at a glance, mentions as familiar things (though not yet so to the reader) objects to be described hereafter by himself. It is a proof of the unity that pervades all Scripture.

make war against them--alluding to Da 7:21, where the same is said of the little horn that sprang up among the ten horns on the fourth beast.