Slytherin’s wand contained a fragment of Basilisk horn. It was passed down through the generations to Gormlaith Gaunt, from whom it was stolen by her niece Isolt Sayre. Slytherin taught it to ‘sleep’ when given a certain command in Parseltongue, rendering it inactive; this secret was passed down along with the wand, but unknown to Isolt until Gormlaith used it against her in her attack on Ilvermorny. The wand was then buried outside the grounds of the school. An unknown type of snakewood tree whose leaves had powerful medicinal properties quickly grew in the spot, seeming ‘testament to the fact that Slytherin’s wand, like his scattered descendants, encompassed both noble and ignoble’. (Pm)
Given that Slytherin himself placed the Basilisk in the Chamber of Secrets (CS9), it seems likely that the fragment of Basilisk horn contained by his wand was taken from the very same creature.