"Now all that's left is to deal with you sir portrait or should I say Salazar Slytherin?" Nick said mockingly and the portrait took on a cold arrogant expression. "How did you figure me out?" the founder asked calmly but with a slight menacing chill. "I told you already didn't I? I am more knowledgeable about souls than you ever were. That ritual of Herpo the foul's leaves a permanent taint on the users soul that anyone sensitive to spiritual energy could spot from miles away. It also didn't help that the magic used to animate portraits wasn't created until about a century after your disappearance from the school." Nick said with a mocking smile.-
Salazar snorted angrily and much to Nick's amazement stepped out of the portrait and stood before him in the flesh. "It matters not I suppose since you won't be leaving this place alive." Salazar said as he pulled out a long wand from his robe that Nick recognized as a branch of the golden apple tree in the Ravenclaw vault. At first glance it looked like Slytherin had full mastery over it but to nick's magical senses it was a very different story.-
The wand was violently rejecting the Salazar who had turned himself into little more than a lich at this point but was being suppressed by the man's massive mana. Oh yes the founder had the most power out of anything Nick had ever encountered up to this point and appeared like a blazing sun to his senses. Nick raised his wand "LUMOS MICARE!" he spoke and Slytherin hissed angrily as Nick blinded him temporarily as an extremely bright burst of light exploded from the tip of Nick's wand. "Finite!" the founder roared but looked confused when his blindness didn't go away.-
Nick smirked and dashed to the side out of the way of a green spell from the founder who silent cast it at his previous position. Nick pulled the Nazgul ring out of Greed and tossed it past Slytherin who had already recovered to a degree and coldly mocked him for not even being able to aim right. Nick grinned viciously "Your projection isn't the target!" he said and it took the founder all of a split second to go pale and turn back to the portrait in a panic. "NO!" he yelled angrily as the ring struck the portrait and immediately wrapped itself around it and went to work.-
"AAAAAHHHHH!" he started screaming in agony as his projection began to unravel as his soul was being devoured by the ring. Nick watched on coldly as this happened since he had been planning this moment since he discovered the true nature of the portrait. See originally he had thought it was merely a cursed artifact that the actual founder left behind that was wrong somehow. But after reading the process of the perfected horcruxes he figured out what was wrong with it , it was a phylactery for Salazar Slytherin himself.-
From that moment on he started to set a trap for Slytherin to force him to use his projection leaving his phylactery unprotected. Salazar Slytherin had nothing to blame for having his phylactery destroyed other than his own arrogance. Much like the diary soul shard he had underestimated Nick far too much and paid the ultimate price for it. He couldn't be blamed though since from the looks of things Nick had cheated to a large degree in order to kill the basilisk. Tying it down and have multiple roosters crow at the same time didn't really take much skill in magic after all.-
Unlike with the horcruxes it took quite a long time for the phylactery to be destroyed by the Nazgul ring since it contained a complete soul. Some might think that this victory should have been impossible given Slytherins power and experience but in truth the man made several mistakes that lead to this outcome. The first was already mentioned as his arrogance lead to him underestimating Nick. The second was that his projection couldn't leave the painting for very long as the ambient mana simply wasn't enough to support his existence.-
This meant that by placing his phylactery in such a location he had grown very rusty over the last thousand years since there was no new knowledge available to learn. He had effectively trapped himself in the chamber of secrets until someone came along able to take his phylactery out. The third and final mistake he made was not considering that with his knowledge of souls Nick might have had a way to destroy his phylactery. All of these mistakes together had lead to Salazar Slytherin meeting his end after a thousand years at the hands of a second year student.