The Sandman(John Milton‪)‬

Two men out for revenge. But only one can win.
When an inmate is found unresponsive in his high security cell at Belmarsh prison, he's rushed to hospital for treatment. The prisoner is human trafficker and international fixer Tristan Huxley, a man a lot of people have good reason to want dead. But Huxley mysteriously never makes it to the hospital. He may have plenty of enemies but it seems he has some very powerful friends too.
Huxley's disappearance is bad news for anyone who betrayed him. One of those people is John Milton. Originally assigned to protect Huxley, things turned sour when he discovered the disturbing truth about his old friend's activities. Huxley isn't someone who forgives and forgets. But neither is Milton-when someone deserves punishment, he's compelled to deliver it, no matter where in the world that journey takes him.
A dangerous man is on the loose and Milton isn't the only one on his hit list. But could he be the only one capable of taking him out for good?