Why is Slughorn an expert on Horcruxes?

Question

Why does Slughorn even know anything about Horcruxes, never mind something super-advanced like having more than one? They seem to be a fairly-unadvertised deep dark magic, and he's neither a Dark Magic adept, nor specializes in DADA or Necromancy - he's a Potions guy.

Answer 1

He was no expert.

This is all that we hear Slughorn say about Horcruxes:

• They are a container for a piece of soul
• You split your soul by killing someone
• There is a spell to encase the soul into the Horcrux

That is not much.
Note also that Slughorn does not discuss the technicalities of making more than one Horcrux, he merely discusses the moral implications. I argue in this answer to another question that Riddle takes this as a confirmation that making more than one Horcrux is possible.

Horcruxes are shunned, but not unknown

It is interesting to observe the exchange between Slughorn and Riddle when the latter brings up the topic:

"Ask away, then, m'boy, ask away..."

"Sir, I wondered what you know about...about Horcruxes?"

Slughorn stared at him, his fingers absent-mindedly caressing the stem of his wine glass. "Project for Defence against the Dark Arts, is it?" But Harry could tell that Slughorn knew perfectly well that this was not schoolwork.

"Not exactly, sir," said Riddle, "I came across the term while reading and I didn't fully understand it."

"No...well...you'd be hard-pushed to find a book at Hogwarts that'll give you details on Horcruxes, Tom. That's very Dark stuff, very Dark indeed," said Slughorn.

(Harry Potter and the Half-Blood Prince: Horcruxes)

The first thing to notice is that Horcruxes are so dark that it is immediately clear to Slughorn that this is no innocent interest of Riddle, not something that one accidentally stumbles upon when studying at Hogwarts. On the other hand, though, you can infer from the discussion that Horcruxes are not secret enough for it to be a big surprise that someone would ask questions around that topic.

And then there is the ban on Horcruxes as a topic in Hogwarts:

'But all the same, Tom... keep it quiet, what I've told - that's to say, what we've discussed. People wouldn't like to think we've been chatting about Horcruxes. It's a banned subject at Hogwarts, you know... Dumbledore's particularly fierce about it...'

(Harry Potter and the Half-Blood Prince: Horcruxes)

This paragraph implies a lot of things:

• Horcruxes are well-known enough for people to take issue with them being discussed
• Horcruxes are well-known enough to warrant a ban on it as a topic
• This ban is broken often enough for Dumbledore to be 'fierce' in enforcing it.

My best guess is that something about Horcruxes is morbidly fascinating, so people keep bringing it up every now and then. It's not explicitly stated, but for me it follows from this sentence of Slughorn's:

"It's natural to feel some curiosity about these things... wizards of a certain calibre have always been drawn to that aspect of magic..."
(Harry Potter and the Half-Blood Prince: Horcruxes)