The Collaborative International Dictionary of English v.0.48:

Ascendency \As*cend"en*cy\, n.
   Governing or controlling influence; the state that exists
   when one person or group has power over another; domination;
   power.
   [1913 Webster + WordNet 1.5]

         An undisputed ascendency.                --Macaulay.
   [1913 Webster]

         Custom has an ascendency over the understanding.
                                                  --Watts.
   [1913 Webster]

   Syn: Control; authority; influence; sway; dominion;
        prevalence; domination; dominance; ascendance;
        ascendence.
        [1913 Webster]

WordNet (r) 3.0 (2006):

ascendency
    n 1: the state that exists when one person or group has power
         over another; "her apparent dominance of her husband was
         really her attempt to make him pay attention to her" [syn:
         dominance, ascendance, ascendence, ascendancy,
         ascendency, control]