Tell the Master—construction of the weapon is complete, built as he commanded. All the Nirncrux in this cavern made it possible.
The weapon will ignite the magicka that flows in the bodies of our opponents—flay them from the inside out with the Master's aura. The magicka wielders who face us will have to deplete their stores or face burning alive.
But we all have a little magicka in us. Those without the means to expel it—the brutes who deign to wield magic, or those who simply fail to bring a spell into battle—will perish with great prejudice.
All that's left is to free the Master.