Acromantula venom is a highly valuable fluid that is secreted from the pincers of an Acromantula. Due to the nature of the Acromantula, the venom is virtually impossible to collect from a live specimen. The venom is therefore easier to collect after the death of the creature, but the venom dries out not long after death, so a Wizard must be swift in collecting it. Because of the rarity of the venom, it may fetch up to a hundred Galleons a pint on the open market.
Acromantula venom was the main reason why Horace Slughorn accompanied Harry Potter to Rubeus Hagrid's cabin for Aragog's funeral. Slughorn was able to acquire what is assumed to be a large amount of venom from Aragog before they buried him. Because Slughorn was at the funeral, Harry was able to gain the memory he had been tasked to get by Albus Dumbledore.