"Ascendancy" (dating to 1712) refers to a state of dominant power or influence, formed in English from the adjective ascendant (from Latin ascendentem) combined with the abstract noun suffix -cy.  It stems from the Latin root ascendere ("to climb up", "go up", or "rise"), combining ad- ("to") + scandere ("to climb"), which is also the root for ascend + the suffix -cy, denoting a "state of being" or quality (similar to infancy or supremacy).  The term comes from the phrase "in the ascendant," which originated in 17th-century astrology to describe a planet or zodiac sign that is "rising" on the eastern horizon and therefore has "predominant influence".  It refers specifically to a state of governing or controlling influence (e.g., "gaining dominance").  

According to the Oxford English Dictionary (OED), the earliest known use of the noun ascendancy is in the mid-1700s, with their earliest evidence dating to 1744 in the writings of 'H. Polesworth'.  The variant ascendance appeared slightly earlier, in 1742.  The word was formed within English, deriving from ascendant  + the suffix -ancy.  Ascendant (adjective & noun, around 1386) is the person or thing that is rising/dominant.   Ascendance (noun, 1742) a closely related synonym that sometimes emphasizes the act of rising, while ascendancy usually refers to the state of being on top.  Ascension (noun, around 1315) is often used for religious or formal upward movement. 

The concept of ascendancy—referring to a position of dominant power, influence, or the process of rising to such a state—is vital for crafting compelling narratives by driving character evolution, shaping power dynamics, and building immersive worlds. It dictates the stakes, structural arcs, and thematic exploration of ambition, morality, and change.  

The importance of ascendancy when writing stories:  
Character Arc and Motivation: Ascendancy provides a clear, high-stakes trajectory for characters, often involving a "leveling up" or transcendence of mortal limitations. It enables the exploration of how power shapes—or destroys—a character's psyche.  
Structural Power Struggles: It is foundational to plot, establishing hierarchies and the conflict that arises when a character challenges the established order, as seen in the Path to Ascendancy series.  
World-Building and Stakes: The concept allows for building complex, often "arcane" systems of power, where characters might ascend from mortals to godlike figures. This establishes a "sense of wonder" or looming threat, essential for engaging readers.  
Thematic Depth: Ascendancy allows writers to explore "gray areas" between right and wrong, forcing characters to make difficult choices that define their morality. It can represent the triumph of will, but also the potential loss of humanity as one rises higher. 

Understanding the distinction between ascendance (the process of rising) and ascendancy (the state of being in power) helps authors refine the pacing and thematic focus of their storytelling.