The most significant use of a `Maxima`-modified spell in the series occurs during the Battle of Hogwarts. As Voldemort and his army of Death Eaters prepared to assault Hogwarts Castle, the school's defenders erected a powerful set of protective enchantments. In Harry Potter and the Deathly Hallows, Professor Flitwick, Professor McGonagall, Professor Slughorn, and Molly Weasley work in concert to cast a series of powerful Shield Charms to fortify the castle. They cast `Protego Maxima` in unison along with `Fianto Duri` and `Repello Inimicum`. The combined effect of these spells created a massive, shimmering, magical barrier that enveloped the entire castle, protecting it from the initial magical onslaught of the attacking forces. This barrier was immensely powerful, though it was eventually shattered by the combined dark magic of Voldemort and his followers.
The difficulty of using this modifier is not explicitly stated, but it is implied to be advanced magic. Its only canonical users are highly accomplished witches and wizards, suggesting that a high degree of magical skill is required to successfully amplify a spell to its maximum level without it backfiring. There is no specific counter-spell for the `Maxima` modifier itself. To counter a `Maxima`-enhanced spell, one would need to use a counter-spell or offensive spell of sufficient power to overcome the amplified magic. For instance, the protective shield over Hogwarts was brought down not by a single counter-charm, but by a sustained barrage of powerful Dark Magic.
The word `Maxima` is a Latin adjective. It is the feminine singular or neuter plural form of `maximus`, which means “greatest,” “largest,” or “highest.” This etymology directly reflects the modifier's function: to make a spell the “greatest” or most powerful version of itself.