Horace Slughorn was using it in case Death Eaters came looking for him. He wanted it to be appear like no one was there, or if there had been, they were dead.
Dumbledore knew it was dragon's blood because he came up with the 12 uses for dragon's blood.
Horace
Severus Snape Horace Slughorn
Horace Slughorn.
It is presently unknown what Horace Slughorn's wand make up is, in both what wood it was crafted from, and its magical material core.
Professor Horace Slughorn takes over the position of potions master. Professor Severus Snape becomes the Defence Against the Dark Arts teacher. The following year, Slughorn becomes the Head of Slytherin house after Snape becomes Headmaster.
Horace
Horace, Horace Slughorn.
Slughorn was still the potions master in Harry Potter and the Deathly Hallows
Severus Snape Horace Slughorn
Horace Slughorn.
Horace Slughorn.
Horace Slughorn.
At Hagrid's hut, when Harry, Hagrid, And Horace Slughorn are having a "funeral" for Aragog.
It is presently unknown what Horace Slughorn's wand make up is, in both what wood it was crafted from, and its magical material core.
Retired. Before his retirement he was also a Hogwarts professor.
Professor Horace Slughorn takes over the position of potions master. Professor Severus Snape becomes the Defence Against the Dark Arts teacher. The following year, Slughorn becomes the Head of Slytherin house after Snape becomes Headmaster.
He guessed after Harry told him what happened with the diary. It wasn't confirmed for him until Harry managed to get the real memory from Horace Slughorn. He had a suspicion that Horace Slughorn knew much more about Voldemort than he told and that's why he convinced him to come back to Hogwarts.