In the preceding verse the order of resurrection is mentioned: the first, Christ; then those who are Christ's. Accordingly, in this verse, after then, one would expect a group of people to be named. However, here it does not tell us who, but it says "the end," because those who will be resurrected at the end in their own order are not in Christ.
The end here is the end of all the ages and dispensations of the old creation. It is also the end of the millennium before the new heaven and new earth (Rev. 21:1). In all the ages and dispensations God, on one hand, has been dealing with His enemy Satan and all the negative things in the universe; on the other hand, He has been accomplishing all things for the fulfillment of His eternal purpose. The last of the ages and dispensations will be the millennium, the kingdom age, after which all God's dealings and accomplishments will be fully completed. That completion will be the end, the conclusion, of all God's work. At this end all the dead unbelievers, the unrighteous, will be resurrected unto judgment for eternal perdition (John 5:29; Rev. 20:5, 11-15). They will be third in order in the resurrection.