Unclear; protection — hardens or strengthens magical barriers (most likely)
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 combined with the other two, bolsters the strength of the shield itself against extremely powerful spells; this is unlike minor shield spells, which only block weak hexes and curses. Given its etymology, it is likely this spell causes a solid barrier to be made out of spells such as Protego Maxima.
This spell was cast by Charms Master Filius Flitwick to protect the boundaries of Hogwarts Castle from the Death Eater forces in the late hours of 1 May, 1998, prior to the Battle of Hogwarts that started at midnight the next day. The Death Eaters, on Voldemort's command, started bombarding the magical protection with spells, and it was ultimately brought down by a spell cast by Voldemort. In all probability, it is a spell calling for great power and expertise for its correct execution.
- 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.
- In Harry Potter and the Deathly Hallows, the protective enchantments around the grounds were summoned by Filius Flitwick, and Flitwick alone. In the second part of the film adaptation, Flitwick is joined by Molly Weasley, Horace Slughorn, and Poppy Pomfrey to do so.
- In LEGO Harry Potter: Years 5-7 the protective enchantments around the ground are summoned by Filius Flitwick, Horace Slughorn and Minerva McGonagall .