Oscuasi

Oscuasi is the incantation of a spell that seals shut the mouth of those that falls victim of it, by having skin grow over it in a manner that makes it look they did not have one.

In 1927, Newton Scamander used this spell to track the origins of a feather - a clue found during his search for Porpentina Goldstein - back to Yusuf Kama, whose hat the feather came from.

Etymology
The incantation Avensegium is likely derived from the portmanteau of avens, a Latin adjective for "eager" or "craving", and seguir, the Spanish and Portuguese verb meaning "to follow" (or alternatively from the Catalan seguim, meaning "we follow"). Taken together, Avensegium can be interpreted as "to eagerly follow", which aptly characterises the behaviour of a tracking device.

Behind the scene

 * Given its effect, as well as massive reaction of Professor McGonagall and the other students, it is possible that this was a curse and fell under the category of Dark Magic.

Appearances

 * Fantastic Beasts: The Crimes of Grindelwald - The Original Screenplay
 * Fantastic Beasts: The Crimes of Grindelwald - The Original Screenplay