Lord of both the dead and of the living is one of the names of Jesus (Yeshua‘) that Paul used. Because He died and rose from the dead, Jesus is Lord over both domains.
Lord of both the dead and of the living in Greek:
Kai nekrōn kai zōntōn kyrieusē
Strong’s Concordance numbers: 3499, 2198, 2961
Bible reference: Rom. 14:9
For to this end Christ both died, and rose, and revived, that he might be Lord both of the dead and living. (Romans 14:9 KJV)
Similar names for God are “God of the living,” “Judge of the living and the dead,” and “Judge of all.”