The compound noun desmophulax is formed from desmos (bond/chain) and phulax (guard/keeper), thus meaning 'the keeper of bonds' — a jailer or prison warden. It appears specifically in Acts 16 in the account of Paul and Silas in the Philippian jail. The jailer is one of the most dramatic conversion figures in Acts.
The conversion of the Philippian jailer (Acts 16:27–34) is a masterpiece of Lukan theology. The desmophulax — the man whose job is to keep others in chains — is himself set free by the gospel. When the earthquake shook open every door and loosed every chain, the jailer assumed the prisoners had escaped and drew his sword to kill himself. Paul's cry 'Don't harm yourself! We are all here!' stopped him. The jailer asked, 'What must I do to be saved?' — and Paul's answer is the gospel in one sentence: 'Believe in the Lord Jesus, and you will be saved.' The keeper of chains became a man liberated by grace.