rightfully

[T]he only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others. His own good, either physical or moral, is not a sufficient warrant.

Ne euer will it [King Arthur's sword] breake, ne euer bend. / Wherefore Morddure it rightfully is hight.

The sword is more commonly known as Excalibur.