Fianto Duri is a charm that, when cast in combination with Protego Maxima and Repello Inimicum, conjures up an almost impregnable magical protection barrier. Bodies crossing the boundaries of the barrier disintegrate on the spot, while spells cast at it (if not of enough power to break it) create large, resonating explosions. This spell's effect is most likely one that, when combound with the other two, bolsters the strength of the shield itself against extremely powerful spells; this is unlike minor shield spells that only block weak hexes and curses.
The incantation is derived from the Latin "fiant", meaning "become" and "duri", nominative masculine plural of dūrus", meaning "hard" or "unyielding"; altogether the incantation means "become hard".
Behind the scenes
Although the spell's true effect is unknown (since it was used in combination with other spells in its debut), the spell may cause pre-existing magical barriers to become hard and solid, given its etymology.
Added by Dobby4ever
