Snape swept down the corridor leading to the headmaster's office. Muttering the password to the gargoyle guards, he moved on. Up the spiraling staircase he went, and into the round tower which was until very recently the place where Albus Dumbledore had resided. He gazed out of the window to see several figures entering the forest, their features growing indistinguishable in the growing darkness. Sniffing, he placed the fake Sword of Gryffendor back upon it's place on the wall. He then turned and placed a critical eye upon the room, Redecoration is in order, he thought, looking distastefully at the empty Phoenix perch and the pale blue curtains. But then, he cast a glance at the bottommost empty picture frame on the wall, I shall not be here for long.
Night was quickly falling and the portraits along the wall began to settle down to sleep. The last one remained empty. Unable to even lie down, Snape pulled out his wand.
"Expectro Patronum." He gazed at the gleaming white doe for a moment before she dissolved as his eyes filled with tears. All of this, afterall, was for her. How he longed to see her, to strock her hair, to hear her speak his name, "Severus… my darling Sev…" His every action was to avenge her death, his becoming headmaster, his working with the Order, his efforts to keep Harry alive under Dumbledore's instruction. Harry: that bastard brat, that child who never should have happened. Ah, but all shall be right in the end, he thought, turning again to look out of the window, he's in the process now, completing the mission Dumbledore gave him. And he's working blindly for even he doesn't know the entirety of his fate. But I do— Snape again cast a furtive glance towards the empty picture frame, not even Dumbledore knows that I have my own plans for the boy. For when the time comes—
"Headmaster!" said a voice as the door clattered open. Snape turned to see Amycus Carrow panting in the doorway, his piglike face sweaty and pied.
"You will knock if you wish to speak with me, Carrow," said Snape icily.
"Yes, Headmaster Snape," said Amycus with a sardonic bow, "I've come to inform you that Longbottom and his fellow brats have been delivered the punishment you issued. They're in the forest now." Amycus wiped his face with a stained and dirty sleeve of his robes. "You know those kids deserve worse than a trip into the forest, Snape. I could arrange something—"
"I have already issued a punishment, Carrow, or are you questioning my authority?" Snape bored his black eyes into Carrow's; and Carrow, knowing his thoughts were no longer safe had no choice but to look away.
"No, Snape, Headmaster"
"Now leave my office, Carrow." Amycus Carrow left, shutting the door clumsily after himself.
Snape rubbed his forehead with his hands and once more looked out of the great windows. The moon had just begun to rise over the distant mountains. Feeling that a few hours attempted sleep would be more productive than continuing to stare out of the window, Snape retired to the adjoining bedchamber and shut the door behind him with a snap.
Months passed and Dumbledore's plans were falling into place. Intelligence had reached Snape that Harry had obtained the Hufflepuff's Cup horcrux, and it could be assumed that the boy had already destroyed it with the sword. All that was left was two, well three more horcruxes to destroy.
Daily conversations with Dumbledore's portrait provided Snape with an endless stream of advice.
"Good, good, Snape. The boy is on the right track. Now what I need you to do is—" and on and on. Snape knew he was being used as a pawn in Dumbledore's plan. He knew that to Dumbledore, Snape's life and happiness were last on the list of priority when the future of the Wizarding World was at stake. Snape doubted whether he even knew happiness at all anymore anyway. All he could feel was a thirst for revenge, and following Dumbledore's plan would ultimately lead him to his goal.
It came as a surprise to Snape when one day Dumbledore revealed the entirety of Snape's mission. When the time was right, he was to inform Harry that he was a horcrux and must sacrifice himself in order to bring about the Dark Lord's downfall.
"You must show him your true allegiance to the Order, and that he must do it with the understanding that only his death can save the Wizarding World."
Snape was angry that Dumbledore had waited so long to reveal this to him and was genuinely surprised by this information. Snape had his own agenda to concider, and oddly enough his was becoming increasingly similar to Dumbledore's. The irony was astounding.
The final battle arrived on a dreary and grey day. The fighting was in full swing by the time Snape had moved to the forest, a face among the many other deatheaters circled around the Dark Lord and the Boy Who Lived. Snape glared at Harry, who was the image of his loathsome father. For just a moment, Harry had cast his eyes at Snape, one last look of trust from those eyes, Lily Evans' eyes.
In that glance that lasted no longer than a second, Snape remembered. He remembered how he had chased Harry down, how he had shouted, parried the boy's curses and punches, how they had struggled physically and mentally until Snape had overpowered the boy and had Harry, immobile, pressed against the wall. Snape had forced those eyes which had once belonged to the woman he loved onto his, and Snape had opened his mind. It was then when Snape had Harry helpless and cornered that he revealed Harry's destiny: how Snape had loved Lily, his allegiance to the Order, Dumbledore's plans, and the fact that Harry himself played host to a piece of the Dark Lord's soul. In return Snape received thought from Harry: the destruction of the locket, the cup, the diadem, Neville cutting through Nagini the snake a second before he died, Ginny's last kiss before she ran off towards the battle, and obtaining the resurrection stone. When the exchange was over, Harry slumped down as Snape stepped back, considering him.
"So you've been planning this with Dumbledore from the beginning" observed Harry.
"Yes, Potter"
"And now I have to go and die… so that bit of him… so that Voldemort's soul…"
"Yes, Potter"
Harry had closed his eyes for a moment before standing up. He looked at Snape again and gave him a look of trust and acceptance. Harry then gave Snape a small nod. He donned his invisibility cloak and was gone.
Snape remembered this as he stood in that circle of deatheaters and watched Harry Potter and Lord Voldemort face off. The Dark Lord had noticed the fraction of a second's glance Harry gave Snape and took advantage of the boy's inattention. With lightning speed, the Dark Lord slashed his wand through the air.
"Avada Kedavra!"
Harry crumpled to the ground, a lifeless heap.
The Dark Lord laughed a high-pitched and chilling cackle as the rest of the deatheaters joined in, hooting and cheering with mirth. Snape had reached his moment. He stepped behind the Dark Lord, now prodding Harry with his foot, and raised his own wand.
"Avada Kedavra"
Lord Voldemort glowed green for a moment then fell as well, dead at Harry's feet. The Deatheaters stopped short; staring shocked at the lifeless pile of robes that used to be Lord Voldemort.
"You will come no closer" said Snape as deatheaters all around began to point their wands upon Snape. There was such a deep and powerful force behind these words that no one moved. Indeed every deatheater seemed unable to move.
Snape moved towards the two figures on the ground. The invisibility cloak and Resurrection Stone had fallen out of Harry's robes and now lay on the ground next to him. The Elder Wand was lain a few inches from Voldemort's long fingers.
Snape smiled.
He had, of course, conveniently forgotten one little detail as he showed Harry Dumbledore's plans. In order for Dumbledore's plan to work properly, Harry needed to have been in possession of all three Deathly Hallow relics. Harry had not gained possession of the Elder Wand. The instrument had been in Snape's possession ever since that night on top of the tower.
Snape bent and picked up the stone, cloak and wand. Straightening, he held them aloft, examining them. The deatheaters followed him with their eyes, hate radiating from them.
"You needn't feel and residual allegiance towards the late Lord Voldemort," Snape said as he whipped the Elder Wand through the air. Two Deatheaters fell dead.
"I am your new master, your new Lord."
Lord Snape twisted the Resurrection Stone and a veiled figure appeared; the figure of Lily Evans. Lord Snape slashed the wand again and the remaining deatheaters fell to their knees.
"A new era has begun."