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Expellimellius

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Expellimellius
Incantation:

Eks-PEL-ee-MEL-ee-uhs

Type:

Conjuration

Effect:

Conjures fire (or, possibly, other random destructive effects)

Expellimellius is a spell that causes things to burn on a lower level than Fiendfyre causes. Cho Chang accidentally used it instead of Expelliarmus on Marietta Edgecombe, catching her sleeve on fire, which Harry Potter extinguished.

[edit] Behind the scenes

It´s unknown if this is a spell of its own, or just a random destructive effect. At least, Cho didn´t cast the spell intentionally, so the latter is more probable.