But you did use it. The problem was
the fact your Blessing of Light was sealed by Midgardsormr earlier (at the end of Keeper of the Lake) and thus you could not draw from the Crystals to summon the Blade of Light at will to deliver the final coup di grace to Nabrieles. Hence Moen sacrificed herself by using her own lifeforce (that is, her own body's aether) to power it instead. The device actually did it's job, the problem was gaining the enormous amount of aether to power it. Realizing the auracite was not going to hold the Ascian for long, desperate times called for desperate measures, and she could see no other source of aether other than her own life, which she willingly did.
But as DJMau said, keep playing, it will all become apparent soon enough -
sadly it will not be the last time you see Scions sacrifice themselves like that either
.