"gain ascendancy" is a correct and usable phrase in written English.
It means to gain control or dominance over something or someone. Example: After years of political turmoil, the party was finally able to gain ascendancy and win the election.
With time Rome may gain ascendancy over the north.
It's as if Manhattan has to be disgraced and humiliated for any other borough to gain ascendancy.
The latest GDP update shows that froth still dominates (the flat white reference being a nod to a tech/digital economy that has yet to gain ascendancy).
Dolls have lost their hold, boys have yet to gain ascendancy, and the youngsters' emotions, with nothing to fix on, swirl around one another with dangerous results.
He did not gain ascendancy on a promise of unity and compromise, but by running on a platform of resentment, rage and bigotry.
If the Islamic world is going to enjoy a revival, if fundamentalists are to be tamed, if women are to be employed more productively, then moderate interpretations of the Koran will have to gain ascendancy.
As the English East India Company gained ascendancy in the 18th century, Thomas Christians faced new challenges.
Although Juan Pacheco, marqués de Villena, initially gained ascendancy over the king, others vied for royal favour.
With the advent of national nautical almanacs, the equatorial system, which is better suited to observation and navigation, gained ascendancy.
Instead for all their lack of air power, drones and high-tech equipment, the Taliban are gaining ascendancy.
With the administrative reorganization that started in the late 1970s, Catalonia became a comunidad autónoma ("autonomous community"), and Catalan once again gained ascendancy in eastern Spain.
