The seven angels with the seven last plagues, 1, 2. The song of those who overcome the beast, 3-6. The seven vials full of the wrath of God, 7, 8.
1 And I saw another great and marvelous sign in heaven: seven angels having the seven last plagues, for the wrath of God is filled up in them.
2 And I saw something like a sea of glass mingled with fire. And those who had gained the victory over the beast, over his image, over his mark, and over the number of his name, stand on the sea of glass, having the harps of God.
3 And they sing the song of Moses the servant of God and the song of the Lamb, saying, "Great and marvelous are your works, Lord God Almighty. Just and true are your ways, 'King of nations.'
4 'Who shall not fear you,' O Lord, 'and glorify your name'? For you only are holy. For 'all nations shall come and worship before you,' because your judgments are made manifest."
5 And after that, I looked, and behold, the temple of the tabernacle of the testimony in heaven was opened. 6 And the seven angels came out of the temple, having the seven plagues, clothed in pure and white linen, and having their chests girded with golden girdles. 7 And one of the four living beings gave to the seven angels seven golden vials full of the wrath of God, who lives forever and ever. 8 And the temple was filled with smoke from the glory of God and from his power, and no man was able to enter into the temple until the seven plagues of the seven angels were fulfilled.
Matthew Henry Commentary - Revelation, Chapter 15[➚]
[v.3-4] - Reference, Deuteronomy 32:4.
[v.3a] - "Great and marvelous are your works" - Reference, Psalm 111:2, 139:14.
[v.3b] - "King of nations" - Quoting Jeremiah 10:7. Originally, the text read as, "King of saints." From the Pulpit Commentary: "The reading of the Textus Receptus, ἁγίων, 'of saints,' is certainly incorrect. It does not appear in any Greek manuscripts, but was inserted by Erasmus to represent the 'sanctorum' of his Vulgate, which word, however, is itself a corruption of 'saeculorum,' the true Vulgate reading representing αἰώνων."
[v.4] - Quoting Jeremiah 10:7; Psalm 86:9 (LXX).