'relentless power' is correct and can be used in written English.
It is an adjective used to describe a determined, unstoppable force. For example, "She persevered with a relentless power, never wavering in pursuit of her goals."
The glacier advances, inexorable, crushing everything in its path with sheer relentless power.
A clenched fist as he left the scene augmented the sense of relentless power he is cultivating.
Stewart Lee's Comedy Vehicle (BBC2) thrilled with its anger, and the Corbyn cat/flag diatribe still has relentless power.
As the film chronicles with relentless power, Syria, outside the family's miserable shelter, has fallen into chaos.
Julia Wolfe's music is distinguished by an intense physicality and a relentless power that pushes performers to extremes and demands attention from the audience.
And in "Nonviolence," when the chorus quotes a Hindu text calling for kindness and renunciation, the music has a slashing, relentless power.
The action is deceptively simple but relentless, evoking power games and occasional indifference to routine slaughter.
So although the Hungarians scored twice in the first eight minutes they were steadily worn down by the opposition's speed, skill and relentless attacking power and were beaten 3-2.
Not only were they subjected to low wages and poor working conditions by newly relocated US corporations – and, as consumers, to the relentless marketing power of Walmart, Coca Cola and the rest – but the major fringe benefit of cutting corruption remained illusory.
My stays in Poland introduced me to otherwise empty-handed activists who faced off against Nazis, Communists, and now, capitalism, with relentless personal power.
While Lisbeth is smart, fast, and relentless, her powers are just a slight stretching of the possible.
