The Mithril Skateboard is a skateboard Scott wins after "defeating" Lucas Lee in Volume 2. The board is apparently stronger than others and has the following stats: +4 Speed, +3 Kick, +1 Will. Despite its good stats, Scott is unable to use it since he opted out of the Skateboarding Proficiency back in grade 5 (in place of Longsword Proficiency, as we learn later). The board floats in midair for a moment before vanishing since Scott can't take it.
Mithril
Mithril is a rare non-existing element often mined and smithed into high-leveled and powerful weapons and armour in most Role Playing Games, including World of Warcraft and the Final Fantasy series. The concept of Mithril was first coined in JRR Tolkien's "The Hobbit" when Bilbo Baggins is given a Mithril vest by the Elven King which is impervious to all cuts and piercing. This item was passed on to Frodo Baggins in Lord of the Rings: The Fellowship of the Ring.
In the Game
In Scott Pilgrim vs. the World: The Game upon defeating Lucas Lee the Skateboard appears and as part of the joke the player cannot get it.