This is a beautiful story - a very clever premise that fits so well with canon, and superb characterization of the two men. Snape/Slughorn is one of my favourite rare-pairings, so I was both delighted to see this written in the first place, let alone written so well!
I love the way you have portrayed Slughorn here - good-humoured yet perceptive, aware of his own cowardice yet caring and faithful, forward in his regard for Severus yet quite sweetly hesitant. This line in particular seemed to sum him up very well:
He tested the tip against one plump thumb, crowing with delight when it drew blood and sucking his thumb like a toddler.
The plot concerning the sword and the mystic ruby was also very well woven, and I particularly enjoyed Severus letting the baby unicorn nuzzle him in the forest with the words, "Yes, yes, you are repellent, aren't you?" Very poignant, and also very sweet that at the same time he thought to collect some unicorn hair for Horace.
There are so many wonderful lines and moments in this story I can't quote them all, but suffice to say that it's going firmly into my 'memories' section as one of the loveliest fics I have read for a long time.