The Phantasmal Fog is a type of colored fog appearing in the 3DS version of Mario & Sonic at the London 2012 Olympic Games. It has the ability to shape itself into clones of anybody it encounters. It is also known to increase the strength of others by having them absorb it.
Hundreds of years ago, the fog was once rampant, but was sealed away inside a magic urn and kept at Stonehenge.
In 2012, Bowser and Dr. Eggman located the urn, and Bowser had a Magikoopa break the seal on the urn by using the mystical energies at Stonehenge combined with its own magic. The seal is broken, and Eggman enslaves E-123 Omega to retrieve the urn.
As revenge for not receiving invitations to the London Olympics, Bowser and Dr. Eggman unleash the fog upon London by using various fog-spreading machines placed across the city. With the thick, colored fog in place, the London Olympics cannot be held.
After the protagonists defeat various fog clones and destroy the fog machines, the city is cleared. However, Bowser and Eggman use the machine in their lair at Big Ben to spread the across the city once more. The fog is now multicolored and is more powerful than ever. Inside Big Ben, where the fog is strongest, Bowser and Dr. Eggman absorb some of the fog, which increases their strength. However, this is not to last; by the time Sonic the Hedgehog, Mario, Miles "Tails" Prower, and Luigi reach them, the fog across the city has cleared except for that inside Big Ben. The doctor and Bowser attempt to defeat the four at Olympic events, but fail. Sonic then destroys the main fog machine, and Mario seals the fog inside the magic urn once more.