His oath forbids him to.

The moment he removes his mask from his face his power is gone, and neither the devil nor the good angels will obey him any longer." "That is true," returned the old man solemnly.

"When he likes he can make himself invisible.

I know it.

He has always escaped pursuit even when the whole country was out after him, and when they thought they had him fast he always disappeared in the earth or in the air.