only 1 use
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- His long right arm shot out, and he grabbed Sam's wrist: his fingers were like a vice; slowly and relentlessly he bent the hand down and forward, till with a cry of pain Sam released the sword and it fell to the ground; and all the while Gollum's other hand was tightening on Sam's throat.4.9 -- Book 4 Chapter 9 -- Shelob's Lair (91% in)
There are no more uses of "relentless" in The Two Towers.
Typical Usage (best examples)