The Mask of Strength is an item that further increases the amount of damage done to a colossus. To receive it, one must defeat six colossi in Time Attack on normal difficulty. It stacks with the Cloak of Force, but the Mask of Power cannot be equipped at the same time.
Dormin says the following upon retrieving the item:
That is the "Mask of Strength"... By wearing it, thou shalt be able to inflict deeper wounds upon a colossus...
The mask bears a striking resemblence to the mask one of Lord Emon's men wears.