Welcome to the first lesson, where we will cover the basics of Occlumency and some of its practitioners. This is necessary to know before we delve into practical work.
Occlumency is, in short, an act where you magically close your mind against any external magical penetration, most commonly Legilimency. That way, a person can prevent anyone from accessing their thoughts, feelings, memories or from influencing them. Sometimes, one can even prevent themselves from being temporarily possessed. If you practice this art, and practice it well, you are known as an Occlumens.
This branch of magic is very difficult to learn, so you will not see it in normal wizarding school's timetables. Part of the reason why it is this difficult to learn, is because for it to work, a person must completely empty themselves of emotion. Humans are emotional beings by nature, so it takes some time for us to learn to let go of emotions. Occlumens needs to make the mind blank and empty, quiet, like the calmest ocean. Those users, who are more advanced, can try and suppress their thoughts, emotions and memories completely or even change them to a degree. The latter is most useful, because it is not as obvious that Occlumency is being used and a Legilimens might take the changed memory as truth.
Occlumency obviously uses a great deal of magical power, and willpower, together with a high degree of mental and emotional discipline. However, if you can master it, you can even resist such things as the Imperius Curse and Veritaserum.
The Boy Who Lived
In his fifth year, Harry Potter was taught Occlumency by Severus Snape, though with little success. Briefly used Occlumeny against Lord Voldemort, with small effect. He will never achieve true mastery of it, due to his tendency to be very emotional.
Death Eater/Order of the Phoenix Member
An accomplished Occlumens, Severus Snape often utilized Occlumency to spy on Lord Voldemort for Order of the Phoenix. Later taught Harry Potter in this subject.
Was taught by Bellatrix Lestrange to conceal the fact that his mission was to kill Albus Dumbledore. Succeeded in shutting off Severus Snape by burying his compassion.
Order of the Phoenix Member
Probably used Occlumency to hide the fact that Sirius Black, James Potter and Peter Pettigrew were Animagi throughout his tenure at Hogwarts.
Follower of Lord Voldemort
Possibly used Occlumency when she lied to Lord Voldemort about Harry Potter's death in The Forbidden Forest.
Taught her nephew, Draco Malfoy, Occlumency. She was highly likely taught by Lord Voldemort.
Now, the question that is often asked in this class is; Can Muggles learn Occlumency? Main reason for this query is that there appears to be no magical spells, or anything magical at all really, that would help Occlumency take place. Only strict control over one's emotions, which falls under mental practice. This has been a point of contention in the wizarding world for quite some time and still remains unanswered, mostly due to the magical world possessing very little information on detailed, genetic, distinction between Muggles, Squibs and Wizards. Especially so in regard to Occlumency, since it is a very obscure branch of magic in general.
However, my personal belief is that yes, Muggles have the capacity to learn Occlumency, but only if they are able to sense the magical intrusion on their person. However, this is rarely the case. Occlumency is more of a meditative and mental practice, where personal mind control and self-awareness is the key to a successful repellence of a Legilimens. Therefore, someone who is highly experienced and skilled in this area, Muggle or Wizard, has a better ability to detect and resist the intrusive consciousness. On another hand, someone who is good at visual imagination and visualizing different scenarios for a longer time might be able to drown out the thoughts that Legilimens wants access to with inconsequential or false memories.