Taboo

"Fear of a name increases fear of a thing itself."

- Albus Dumbledore to Harry Potter in 1992

Description
The Taboo curse is a powerful spell which designates a word as a key to revealing the speaker's location. As such, it works in a similar manner to the Trace spell, but rather than being triggered when any magic at all is used, it is triggered by the utterance of a specific word. It works regardless of the age of the speaker and disables some weaker protective magical enchantments. If the speaker is in a building protected by the powerful Fidelius Charm, for example, it may reveal the general area around the speaker's house, but will not break the charm. This curse controls individuals by limiting their freedom to speak, and thus causes great stress, terror, and frustration. Because of this, it may be considered to be Dark magic.

Use in the First Wizarding War
Some have theorised that the practise of saying "You-Know-Who" instead of Voldemort's proper name might have began when he used a Taboo on his name during the First Wizarding War. This is unlikely, because Dumbledore encouraged people to use the proper name Voldemort so as to not fear the name. He would not have encouraged this if saying the Dark Lord's name would have endangered people. The Taboo's use during the Second Wizarding War seems to have been novel. It was particularly effective in capturing members of the Order of the Phoenix, as well as the D.A. The practise of saying "You-Know-Who" instead of Voldemort is most likely caused by memories of the extreme fear and panic hearing his name would inspire, and a desire to avoid arousing those bad memories.

Implementation
Sometime after seizing indirect control of the Ministry of Magic in 1997, Voldemort made his own name Taboo. Thus, whenever his name was spoken aloud, Death Eaters and Snatchers were alerted to the location of the speaker. Since the only individuals who dared speak Voldemort's name were members of the Order of the Phoenix or their allies, the Taboo allowed Voldemort's followers to track down their enemies. Anyone who dared to speak the name and break the Taboo would be hunted down as criminals. Hermione Granger broke the Taboo once while the trio were at Tottenham Court Road, which led two Death Eaters (disguised as construction workers) to their location. Harry Potter broke the Taboo while they were camping during the hunt for Voldemort's horcruxes, leading to their capture by Snatchers. Kingsley Shacklebolt broke the Taboo and had to fight off several Death Eaters before going on the run, which is what alerted the Order of the Phoenix to the Taboo's existence.

The anti-Voldemort radio programme Potterwatch, which was run by Order members and sympathisers, took to referring to Voldemort as "Chief Death Eater" as one of their methods of avoiding detection.

It is also possible that Voldemort invented the Taboo curse, as it seemed that no one was aware how his Death Eaters were able to track people who spoke Voldemort's name.

Known triggers

 * Death Eaters Antonin Dolohov and Thorfinn Rowle used the Taboo curse to trace Harry Potter, Ron Weasley and Hermione Granger to a café on Tottenham Court Road shortly after they Disapparated from Bill Weasley and Fleur Delacour's wedding. The trio managed to escape. Although they did not know about the Taboo for some time, they stopped using Voldemort's name at Ron's request.
 * Death Eaters also used it to corner Kingsley Shacklebolt, but the Auror and Order member fought his way free and went on the run.
 * Harry Potter accidentally broke the Taboo in the spring of 1998, leading to the trio's capture by a gang of Snatchers led by Fenrir Greyback and subsequent detention at and escape from Malfoy Manor.
 * In Harry Potter and the Deathly Hallows: Part 1, Xenophilius Lovegood purposely triggered the Taboo in order to lure Death Eaters to his house to capture Harry.

Possible triggers

 * The Taboo curse might have been responsible for the capture of Dean Thomas and Griphook, and the deaths of Ted Tonks, Dirk Cresswell, and Gornuk, at the hands of Snatchers.

Defence against the Taboo
The Taboo, when triggered can break normal defensive enchantments (invisibility, etc.) such as when Harry, Hermione, and Ron were captured by the Snatchers. However, when inside the protection of very powerful enchantments such as the Fidelius Charm, the protection seems to remain unchanged. This may be why the trio were protected at 12 Grimmauld Place, even though they repeatedly used Lord Voldemort's name while hiding there in 1997, as whenever Harry looked outside the window, he could see Death Eaters lurking around the place. So it can be made sure that the Taboo does not break a Fidelius Charm, but directs the person to near the location, if the person does not know the address or is Secret Keeper.

Appearances

 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1 

Notes and references
Tabou