Arania Exumai

Ron: "Know any spells?"

Harry: "One, but it's not powerful enough for all of them."

Ron: "Where's Hermione when you need her?"

Harry: "Let's go! Arania Exumai"

— Harry saving himself and Ron from the Acromantula^[src]

Arania Exumai is the incantation of a spell used to blast back acromantulas or other large spiders. If the spell is used on anything but a spider it will produce a large black scorch mark, and thus it is not recommended for use on humans^[1].

Known uses

When Tom Riddle had to frame Rubeus Hagrid for opening the Chamber of Secrets and tried to attack Aragog in 13 June, 1943, he used this spell against the acromantula after forcing him to leave the box via Cistem Aperio.

50 years later, Riddle showed this memory to Harry Potter via his diary, and Harry would learn this spell for his own usage against Aragog's colony of acromantula in the Forbidden Forest. He used it twice, first to blast away an acromuntula in order to clear the path back into the Flying Ford Anglia, second to save Ron Weasley from another acromantula that got him by the neck.

Etymology

From the Latin araneus, meaning “spider”, and exuo, meaning “I lay aside”.

Known Practitioners

•Tom Riddle •Harry Potter •Hermione Granger (Harry Potter and the Forbidden Journey)

Behind the scenes

The spell was created for the film adaptation of Harry Potter and the Chamber of Secrets and does not appear in any of the novels.
Tom Riddle's use of this spell released a simple white blast of light that missed Araggog but left a scorch mark where the spell narrowly missed him.
Harry Potter's use of this spell released a beam of blue-white light that kind of carried the acromantula away before it was blasted out of sight.
Also while Harry was successful at saving Ron from the acromantula that had seized him, Ron was also caught by the light of the spell in the process, therefore it should have left a scorch mark on him.
In Harry Potter and the Forbidden Journey, Hermione Granger uses this spell to fend the acromantulas off from the riders. After Hermione casts Arania Exumai, it causes a bright flash. This is where the pictures are taken. ^[2]

Appearances

References

[1] Harry Potter and the Chamber of Secrets (film)

[2] Harry Potter and the Forbidden Journey

[1]

[2]