The seventh king is the last of seven kingdoms which oppressed Gods people through history and follows upon the last terrible beast shown to Daniel (Dan. Dan. 7:7, Dan. 7:19-20). The seventh king is the king which had not yet come at the time of Johns vision (Rev. Rev. 17:10+). Concerning the seven heads which are seven kings, John is told, They are seven kings. Five have fallen, one is, and the other has not yet come. And when he comes, he must continue a short time (Rev. Rev. 17:10+).1 The seventh king is said to be future to Johns day, but when he arrives, he will only continue a short time. This probably corresponds to the relatively short period between the rise of the Antichrist and his initial demise by a fatal wound (Rev. Rev. 13:3+ cf. Zec. Zec. 11:17). Thereafter, he is revived as an eighth head or king (Rev. Rev. 17:11+). In our diagram of Symbols of Kingdoms, we have drawn the seventh king within the sixth king because it is evident from the revelation given Daniel concerning the fourth beast (which corresponds to the sixth king) that after its initial appearance and rule, the kingdom of the fourth beast fragments but continues to exist until the time of its ultimate overthrow to establish the Millennial Kingdom (Dan. Dan. 2:35, Dan. 2:44-45). Therefore, in some sense, the seventh king is a continuation of the sixth king, represented by the terrible beast . This understanding requires the emergence of an empire at the time of the end which is in some way a continuation of the Roman empire of Johns day (Dan. Dan. 2:40-44). The seventh king is related to the eighth head, the Beast. The beast that was, and is not, is himself also the eighth, and is of the seven, and is going to perdition (Rev. Rev. 17:11+). It appears the seventh king, having received a fatal wound, enters the abyss [#13-#17] (Rev. Rev. 17:18+) prior to his revival as Beast (Rev. Rev. 13:3+, Rev. 13:14+). This view recognizes that the passage which describes the relationship between the seventh king and the Beast (the eighth head), employs the phrase, that was, and is not, which is associated with the wounding and subsequent ascent of the beast from the abyss (Rev. Rev. 17:11+ cf. Rev. Rev. 17:8+). See #16 - Beast. See Symbols of Kingdoms.
1 Concerning inaccuracies in the KJV and NKJV, see commentary on Revelation 17:10.