A different beggining

"Now that you have spent the better part of a month covering the theoretical groundwork, the time has come for your occlumency lessons to take on a much more practical aspect," Professor Croaker addressed his four students in his customary cold, measured voice(Professor Croaker is a retired unspeakable). "In this lesson, I shall attempt to penetrate your minds. I will not lie to you – the sensation is anything but pleasant and you will most likely fail to repel me in your first attempts. It is nevertheless something which you should experience first-hand." He paused, before continuing, "I will give you a few minutes to prepare your defenses and shift any, well, sensitive memories deeper. I promise not to go beyond the surface. Keep in mind that I am being extremely generous. No true adversary would give you such an advantage. Now then, you first, Mr. Nott."

Theo stepped forward with slight hesitation. His mental faculties were presently engaged in both attempting to suppress some of his more embarrassing memories deeper within his consciousness and at the same time reviewing the theoretical techniques he had so far studied. He quickly became aware that such a division of resources meant he was not achieving meaningful success in either task. Sensing that the grace period allotted by his professor was close to expiring, he deployed a rudimentary but surefire defense which would buy him time for a proper counterattack. He would have to occupy his thoughts with something algorithmic and repetitive, yet mundane enough so as to not tie down too much of his attention. The Fibonacci sequence. Yes, that would do. He started counting off the numbers, 'One. One. Two. Three.

"Legilimens!"

Theo instantly became overwhelmed by the sensation of another consciousness entering his mind. To one so used to being the sole occupant of his own mind, the feeling was akin to being forced into a very narrow space. The foreign presence was overpowering his own consciousness. It was suffocating him. After a brief instant, which certainly felt much longer, he finally managed to muster the willpower and focus to raise a mental shield. His magic surrounded his consciousness, bringing with it a freeing, empowering sensation. It was not to last, however, as Snape's assault continued on its own momentum, destroying the shield. Draco winced at the sharp pain he felt behind his temples. He realized, though, that the experience could have been much more painful. Rather than smash through the shield with brute force like a magical sledgehammer, Croaker's strike was surgical, sufficient to advance him beyond the shield and at the same time damaging enough to destabilize it and make it collapse on its own. Such precision, speed, and economy of force and focus were truly the mark of a master legilimens.

Recovering from the jolt of pain, Theo attempted a second response but found he was unable to call forth another shield. Croaker had placed some kind of suppression barrier against him and was now shuffling through his unguarded memories. Draco's senses were inundated with flashing glimpses. Images. Sounds. Tastes. The rush of the wind during a game of quidditch. The exquisite flavor of last night's dessert. Snape did not linger in any one of the memories; he was quickly switching through them as if flipping through the pages of an imaginary book. Which each passing second, his control and reach grew until it became evident that Theo had been resigned to a passive witness of the thorough violation of his own mental space.

And suddenly, it stopped.

Theo jumped back, and began gasping for air. Even though nothing physical had actually constrained his breathing, he still felt the suffocation sensation of Croaker's intrusion; thankfully, it was now quickly fading away. "Not particularly impressive, but you did raise a shield. That's as good as you can hope to get with your first attempt, Mr. Croaker. And I see you are experiencing the psychosomatic effects. Help yourself to a calming draught from the cabinet," Croaker delivered his assessment. He then called for the other students. Neville and Daphne feared little better, in fact the latter emerged particularly shaken. When at last Harry's turn arrived, he stepped forward cautiously.

"Sir, may I ask a question?"

"If you must, Mr. Potter-Black."

"I was wondering, just how severe can the psychosomatic effects be … I mean, theoretically, that is…"

"At the hands of an amateur legilimens, you may very well end up dead or in the mental ward at Mungo's. Those who have little understanding of the mind usually barge in and overload their adversaries' shields with sheer power, after which they damage or completely destroy many memories until they find the information they seek. The strain that this produces on the nervous system can cause unimaginable pain rivaling that of the cruciatus, and eventually either brain death or cardiac arrest. Of course, even if one survives that, the damage to the memories and personality of the victim far too often result in insanity."

Harry shuddered at that.

"Come now, Mr. Potter-Black. You have nothing to fear. It wouldn't do to damage your mind; for one, all my work will have been for naught. Now prepare to defend yourself!"

Harry closed his eyes so as to focus better; free from the need to maintain situational awareness, he was able to better manipulate and concentrate his magic and raise his shield. He kept his eyes closed, avoiding Theo's mistake – after all, the book had clearly stated that breaking eye contact makes a legilimens's task exponentially harder. With his shield up, he set about creating a second precaution, more akin to a tripwire than a barrier. No sooner had he finished than Croaker started the assault:

"Legilimens!"

It was instantaneous. The outer ward immediately registered the invasion, collapsing soon afterwards, its function having been served. Sensing the start of Croaker's main assault, Harry tried to strengthen his fledgling shield by visualizing a strong defensive wall. It was a shortcut, designed to focus the will and intent driving one's magic into the task at hand. It was useless; his feeble barrier shattered with a single surgical strike from his professor.

With his defense gone, Harry experienced the most unpleasant sensation he could remember. He had of course read the description in his occlumency textbook, yet nothing could prepare him for the actual feeling. It was even worse than the time Sirius had taken him for side-along apparition. The feeling of being invaded, of having the very borders of one's self-being penetrated. Of losing the last bastion of privacy. Losing the coherence of thought. Losing control.

Before he could even become consciously aware of it, an angry storm had replaced the feeling of claustrophobia; now it was ushering a veritable tide of magical power. And with it came a feeling of freedom; the constricting pressure abated at last. The next thing Harry realized was that Croaker's assault had paused; he quickly deduced that this state of affairs would not continue for long, as his professor was most likely trying to adjust his approach to Harry's newly found resolve. His suspicions were proven correct when he sensed the foreign presence again on the very periphery of his consciousness. The new assault was different, however; if the last one could be described as blow of a fist, this felt more like a sledgehammer. And it was fast. Soon, it would overwhelm him. Crush his defenses. Tear his thoughts asunder. Violate his mind…

"NO!"

Not again. Never again! The surge of magical power reached a crescendo. Harry instinctively knew what to do. With all his will and concentration, he thrust his magic into a counterattack. A rudimentary mind probe. There was no grace, no technique – but it was damned effective. It crashed directly into Croaker's mental projection, shattering it to pieces. Harry felt the professor's surprise. Even a master like he would need a moment to switch from offense to defense, and in that infinitesimal instant, he was vulnerable. Instinctively, Harry knew he would not get another chance.

With unabating resolve, he pushed forward through the mental bridge unopposed. The momentum of his attack propelled him past the perimeter of Croaker's consciousness and into his thoughts. Feelings and images rushed around him, too quickly to make out. No matter. He was in control now. He wielded power over him. It felt… immensely satisfying. And his instinct drove him ever onward, to the center of his victim's mind. Almost there.

When Harry awoke a few moments later, he was surrounded by the worried faces of his peers. "Are you alright, Harry?" Lyra asked, visibly agitated,there was dad and mum too what happened?.

"I…I…" He pressed his hand on his forehead. "Headache."

"Take this, Mr. Potter-Black." Croaker offered him a bottle of potion. When Harry looked up at his face, however, he started. Blood was flowing in a thin stream from Croaker's nose!

"Professor, I'm so sorry! I didn't mean to! I –"

"It is nothing, Mr. Potter." A swish of his wand and the injury was gone. "In fact, I must commend you. That was a perfectly timed albeit crude legilimental attack."

"I don't know how it happened," said Harry, trying to recall exactly what occurred, "Something came over me. I-I was acting on instinct."

"Indeed. An instinct which usually takes years to develop. And yet here you are, an utter novice, acting upon it."

"What happened exactly, Professor?" The question came from Theo.

"My initial attack was somewhat subdued, Mr. Nott. Mr. Potter-Black reacted with a shield far stronger than needed to hold it. Consequently I stopped to reinforce my probe. In doing so, I committed the basic error of neglecting my defense. I admit I underestimated Mr. Potter-Black's ability to exploit that flaw. The window of opportunity is very small, and very few can even recognize it. But exploit it he did, and to great effect. His attack, though unrefined, was very concentrated and shattered my own before penetrating my inactive defenses. I believe you saw its psychosomatic manifestation. I had to distract him, to break his focus before I could counterattack and expel him." Croaker turned to Harry. "And you Mr. Potter-Black, have joined the very short list of people who have carried out a successful legilimental strike on me."

Harry's friends looked on him in amazement. "I am flattered, Professor," Harry replied, "but I fear you give me too much credit. For one, I don't remember raising a shield."

"I never planned to attack either, Professor," said Harry, "it just, well, happened." He was struggling to recall the details, but he simply knew that was the necessary course of action. What's more, a part of him actually wanted to strike, to invade Croaker's mind and rip apart his thoughts. He struggled to explain this mysterious bloodlust, although its presence somewhat troubled him.

"Precisely, Mr. Potter. It is this instinct that I am trying to explain." Croaker closed his eyes for a moment, visibly deep in thought. "It is not unheard of," he continued in a lower, measured tone, "for some wizards to have a natural gift in the mind arts. Many may never become aware of it, if they are fortunate to spend their entire lives without an attack on their minds. And yet should someone try to take advantage of this apparent defenselessness, the unfortunate legilimens is in for a most unpleasant and painful surprise. In fact, there are cases where it was precisely such an attack that led the gifted wizard to become aware of his talent and later become very accomplished in the mind arts. Whether the same has happened today remains to be seen." 

Sirius and Cassie also taught their students went into in-depth magical theory on controlling the power of one's spells, something called the String and River Theory which was discovered by Cassopeia Black,their mums namesake and head unspeakable of her time. Basically, it was talking about only allowing a small amount of magic to flow, the bare minimum for spells, versus the river, which was overpowering a spell.Everyone benefited with this but Harry and Lyra benefited the most due to them constantly overpowering the spells.