Long ago, at the conclusion of the battle with King Umbra, he had cursed her staff with his dying breath to her unknowing.
While his dark magic was not strong enough to effect the goddess of the sun or the moon, the day would come when his shadow would happen upon a more vulnerable target.
Just the day before Twilight was to be crowned a new princess of Equestria, Celestia had gifted her this staff.
Sensing the vulnerability of the staff's new wielder, King Umbra's curse went to work. Within a minute of being in its presence her mind was not her known.
Twilight craved power, control, and the downfall of Equestria under the control of King Umbra's shadow.
While Celestia slept that night, Twilight crept into her chambers, however she had not the power to kill the sleeping princess.
Instead she settled upon a spell to lock her mentor into an eternal sleep, granting her the time to build her own power. Once the deed was done she stole Princess Celestia's Radiant Armor to solidify her control.
Princess Luna happened upon this incident at its conclusion, recognizing the evil aura that warped Twilight's mind she jumped to her sister's aid.
The combined might of Twilight and the cursed staff proved to much for Princess Luna, and she was dealt a mortal wound.
Their conflict had attracted the rest of the castle's guard, yet Twilight's power had been drained from the fight with the powerful goddess of the night and she was forced to retreat or risk capture.
Luna, who would not be felled so easily had survived her injury, however was not in health to pursue the possessed unicorn.
As a last resort she summoned the only other unicorn she had known to rival Twilight's power in which she would lend her Lunar Staff and Armor.