8 Sided Dice

Eight-sided dice are a platonic solid known as an octahedron. They are most commonly used as a part of any role-playing game that makes use of a standard 7-dice set, this including games like D&D, Pathfinder, Earthdawn, and Call of Cthulhu. 

The oldest d8 we've been able to track down dates back to 1888 where it was used as a poker die.