The Mask of Power further increases damage to a colossus, it is unlocked by defeating 12 colossi on Normal Time Attack. It stacks with the Cloak of Force, but the Mask of Strength cannot be equipped at the same time.
Dormin says the following upon retrieving the item:
That is the "Mask of Power"... By wearing it, thou shalt be able to inflict more grievous injuries upon a colossus...
- This mask bears a striking resemblence to the mask one of Lord Emon's men wears.