"In ascendancy" suggests a state of rising or increasing in power, popularity or influence. Some synonyms for this phrase could be "on the rise," "gaining momentum," "increasingly dominant," "growing stronger," or "becoming more prevalent." Other words that could be used to convey a similar meaning include "ascendant," "dominant," "supreme," "prevailing," "commanding," and "flourishing." When describing a person or group in this context, one might use terms like "powerful," "influential," or "authoritative." Whether referring to a trend, an ideology or a particular individual, there are many ways to describe the ascent of a force or concept, depending on the specific context and intended effect.