AN: This was supposed to be a short 100 word drabble for a prompt at fic-promptly, but it rapidly grew.
The first time Severus presented himself to the Potions Guild he met with a Potions Master that he managed to get on with straight away. They seemed to understand each other easily, and could follow what the other was saying when they went into long complicated theories relating to their research. It was a true meeting of similar minds; they had similar views on not just potions, but magic and life, they really had a lot in common.
Gaining his potions mastery was one of the best moments of his life, for him it meant he would have unrestricted access to the guildhall, the only place where he was able to meet the other Master. He would have expected the other man to have a portrait at Hogwarts, somewhere in the dungeons perhaps. It wasn't the case though; the only surviving portrait of Salazar Slytherin was kept hidden within the potions guild.
It didn't matter to him that the man that had fast become his best friend was a portrait, or that he had been dead for over a millennium. Salazar was intelligent and cunning, and they were both quite aware that they had acerbic personalities and accepted each other. As a former Slytherin student Severus looked up to the founder of his house and valued any advice the elder Potions Master gave him.
He wouldn't say that their friendship was without strife, especially when Salazar found out about the Dark Lord and the actions being carried out in his name. Upon turning from the Dark Lord as a servant and taking up a position at Hogwarts their friendship had become less strained. His promise to Salazar to protect and guide the Slytherin students and try and stop them from walking the same path he had was one that he had made gladly.
Over the years their friendship had only grown, much to the curiosity of the other Potions Masters that he spent so much time talking to the most taciturn portrait to grace the halls of the guild. By the time that the final battle came Severus had set all his affairs in order, knowing that his survival once he had been revealed as a spy was unlikely. The Dark Lord would surely kill him for his treachery, or maybe a member of the light side would kill him to avenge Albus. Severus didn't care what side of the war dealt the final blow to him, as long as he had the opportunity to tell Potter what needed to be done.
Severus knew that his death was coming, he had straddled the line between both factions of the war for too long; someone was going to ensure his death. The protection of the students would fall to Minerva and hopefully Potter would end the Dark Lord for good. His part in the war was over though, and he was glad, he had accepted it. He would get to see his friends again, Lily and Albus, and he could finally meet Salazar; hopefully they could hole up in a lab for a while and work on some of the potions that they had discussed so often.