The Collaborative International Dictionary of English v.0.48:

Ascendancy \As*cend"an*cy\, Ascendance \As*cend"ance\, n.
   Same as Ascendency.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ascendancy
    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]

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "ascendancy":
   Cadmean victory, KO, Pyrrhic victory, accomplishment, ascendance,
   authority, balance of power, championship, charisma, charm, clout,
   conquest, consequence, control, credit, deanship, dominance,
   dominancy, domination, dominion, easy victory, effect, eminence,
   eminent domain, enchantment, esteem, excellence, favor, force,
   good feeling, grand slam, greatness, hold, importance,
   incidental power, incomparability, influence, influentiality,
   inimitability, insinuation, knockout, landslide, landslide victory,
   lead, leadership, leverage, magnetism, majority, masterdom,
   mastery, moment, moral victory, one-upmanship, overlordship,
   personality, persuasion, picnic, potency, power, precedence,
   predominance, predominancy, predomination, preeminence,
   preponderance, prepotence, prepotency, prerogative, pressure,
   prestige, primacy, principality, priority, privilege, purchase,
   pushover, reign, repute, right-of-way, rule, runaway victory, say,
   seniority, skill, sovereignty, suasion, subdual, subduing,
   subtle influence, success, suggestion, superiority, supremacy,
   suzerainship, suzerainty, sway, total victory, transcendence,
   transcendency, triumph, upper hand, victory, virtuosity, walkaway,
   walkover, weight, whip hand, win, winning, winning streak