Protego Diabolica

"Play by the rules! No cheating, children!"

- Grindelwald to the Aurors attempting to flee from his spell

Protego Diabolica is the incantation of a powerful protective dark charm. It allows the user to create a protective ring of black fire around them that completely incinerates enemies of the caster who come into contact with it while leaving their allies unharmed. In 1927, Gellert Grindelwald cast this spell in the Lestrange Mausoleum to test the loyalty of his followers and kill a number of his enemies.

Casting and effects
When cast, Protego Diabolica creates a ring of black fire around the caster. It is possible to walk through the flames unharmed, as demonstrated by Vinda Rosier, Credence Barebone, Queenie Goldstein and Abernathy amongst others, if they are loyal to the caster and mean them no harm. However, Krall, who earlier expressed doubt about Grindelwald's goals, was burnt to ash within a few moments of entering the fire.

Grindelwald was also seen controlling the spell as a weapon by causing tendrils of black fire to extend from the ring and burn his enemies, some of whom were in the process of apparating. The tendrils can be deflected or blocked, temporarily, with the Shield Charm.

Grindelwald was able to use the ring of fire in an attempt to destroy Paris. The fire soon manifested what appeared to be a fiery winged demon within it as it spread and burnt everything in sight, similar to the effects of Fiendfyre. It was so powerful that it required the combined usage of the General Counter-Spell by Newt and Theseus Scamander, Tina Goldstein, Yusuf Kama, Nicolas Flamel and a number of others.

Etymology

 * The incantation comes from Latin "protego", "I protect", and "diabolica", a declension of "diabolicus", meaning "diabolic, relating to the Devil".
 * It is unclear if the translation is meant to suggest “protection from the devil” or “the devil protects.” As the spell is used to divide friend from foe, one can assume that the former translation is accurate, as Grindelwald was dividing himself from those whom he considered “the enemy,“ (also a euphemism for Satan). However, given Grindelwald’s real intentions and his habit of using irony and misleading phrases (such as the phrase “The Greater Good” when he is, in fact, inflicting suffering rather than good), it could very well be the latter.

Behind the scenes

 * In 🇨🇬, the spell creates a "circle of black fire", whereas in the film the flames are bright blue. This is most likely due to the other tones of the scene being too dark for black fire to properly be seen. Therefore, a blue fire might have been used instead to show contrast to these dark tones.
 * The spell's trait of rapidly spreading and growing in size and being able to create bestial entities out of its flames is remarkably similar to Fiendfyre, suggesting the spells are related. Additionally, the spell shares similarities to Bluebell Flames, which can seemingly be made to only damage certain targets and objects, as well as the caster being able to direct its movement to an extent.

Appearances

 * Harry Potter and Fantastic Beasts: A Spellbinding Guide to the Films
 * Harry Potter and Fantastic Beasts: A Spellbinding Guide to the Films
 * Harry Potter and Fantastic Beasts: A Spellbinding Guide to the Films

Notes and references
Protego diabolica Protego Diabolica de2:Protego Diabolica Protego Diabolica Protego Diabolica pt-br:Protego Diabolica