Here via <lj user=snegurochka_lee>'s emphatic rec!
Wow. What a tale! There's so much to love about this story, so very much, from Slughorn's narrative voice to the little details like Snape's boots, brass buckles, not silver, and that bit at the end, when he finally sussed out that it was Severus, and OMG the tension right there!
And then, there are lines of pure beauty and brilliance, like this one... The fellow was witty and intriguing and, while on the older side of such things, precisely the sort of younger companion an old man abroad should make himself foolish over.
I liked the plausibility of Severus' survival and his cover story, Horace's efforts to uncover Benjamin's history and the parts of him that were so reluctant to ... befriend another upperclassman who might yet be a murderer. That line is so eloquent in its despair and in Horace's deep-seated horror at the events in his past - it's just... I found this entire story stunning.