Fantasy Fire by Max Factor is a polish that isn’t the easiest to get your hands on around the world, although it seems freely available here in the UK.  It’s very similar to the slightly mythical ‘Unicorn Pee’ of Estee Lauder, which I’ve never seen in person or had on my nails.

In the bottle it looks purple with a red shimmer and on the nail (on it’s own) it is very sheer and doesn’t seem anything that special.  But, layer it over other polishes and you’ll see what’s been driving people crazy!
So I have a few swatches for you.  First up, over a dark purple base – Nars After Dark.

The red and shimmer of Fantasy Fire really come out in these photos (artificial lighting) and although it looks glittery, it’s pure shimmer.
Next up, swatched over bright blue…

The only way to describe this colour is electric purple and it really looked that bright in person! The base was Barry M Gelly in Blue Grape.  You can see the red shimmer too.  These photos are without topcoat and in indirect daylight.
The downside about Fantasy Fire is it comes in really tiny bottles!  But I would seriously recommend picking up a bottle or three if you see it. It also looks amazing over black, with a red/green shift. 
I got my Max Factor Fantasy Fire at Boots for £3.99, it is more expensive on eBay but shop around and see what you can find.