Relentless (2009 novel)
WordNet
adjective
(1) Never-ceasing
"The relentless beat of the drums"
(2) Not to be placated or appeased or moved by entreaty
"Grim determination"
"Grim necessity"
"Russia's final hour, it seemed, approached with inexorable certainty"
"Relentless persecution"
"The stern demands of parenthood"