There are several points here that need attention.

What is done is the execution of the instruction given in Revelation 16:1. That is, the seven angels have gone and poured out their seven bowls. Better put, all the wrath of God (note the number seven) that needed to be poured out on rebellious humanity has now been poured out.

The verb translated as it is done appears five more times in the present paragraph (translated as were or was or absorbed in other verbs) and so is a key word in the passage. God’s pronouncement, It is done, is the catalyst for other conclusive activity on earth.

The clause used here, It is done, directs our minds to Jesus’ last words on the cross: It is finished (John 19:30). To our ears those two phrases come down to the same thing. In the Greek two very different verbs, with different meanings, are used. In Jesus’ case, the verb used denotes that whatever was foreshadowed in the Old Testament for reconciling sinners to God was in fact accomplished on the cross. In the present case the verb used denotes that a mandate once given has been executed (in this case, seven bowls poured out). The two pronouncements, of course, are definitely connected. Had Jesus on the cross not completed the task God gave him (and that included defeating Satan, Colossians 2:15), the pouring out of God’s wrath on the air would not have had its desired effect.