relent, relentless

1. VERB

If you relent, you allow someone to do something that you had previously refused to allow them to do.     

• Finally his mother relented and gave permission for her youngest son to marry.
  

2. VERB

If bad weather relents, it improves.     

• If the weather relents, the game will be finished today.
  

... cobuild

1. to change one's mind about some decision

2. to become milder or less severe: the weather relented

 [Latin re- back + lentare to bend]

... Collins Essential

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1. relentless - 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"

grim, inexorable, unappeasable, unrelenting, unforgiving, stern

implacable - incapable of being placated; "an implacable enemy"

2. relentless - never-ceasing; "the relentless beat of the drums"

persistent, unrelenting

continual - occurring without interruption; chiefly restricted to what recurs regularly or frequently in a prolonged and closely spaced series; "the continual banging of the shutters"

... http://www.thefreedictionary.com/relentless

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Synonyms: yield, relent, bow, defer(2), submit, capitulate, succumb

These verbs all mean to give in to what one can no longer oppose or resist. 

• Yield has the widest application: My neighbor won't yield to reason. "The child ... soon yielded to the drowsiness" Charles Dickens. 
• To relent is to moderate the harshness or severity of an attitude or decision: "The captain at last relented, and told him that he might make himself at home" Herman Melville. 
• Bow suggests giving way in defeat or through courtesy: "Bow and accept the end/Of a love" Robert Frost. 
• To defer is to yield out of respect for or in recognition of another's authority, knowledge, or judgment: "Philip ... had the good sense to defer to the long experience and the wisdom of his father" William Hickling Prescott.
• Submit implies giving way out of necessity, as after futile or unsuccessful resistance: "obliged to submit to those laws which are imposed upon us Abigail Adams.
• Capitulate implies surrender to pressure, force, compulsion, or inevitability: "I will be conquered; I will not capitulate [to illness]" Samuel Johnson.
• Succumb strongly suggests submission to something overpowering or overwhelming: "I didn't succumb without a struggle to my uncle's allurements'' H.G. Wells.