Before you get your hopes up, the GIGANTIC RAINBOW SKITTLES CAKE THAT CAN FEED UP TO 20 PEOPLE is likely not being sold anywhere near you. Per The Sun, this "[showstopper]" is only currently being sold at U.K. chains ASDA, Morrisons and Sainsbury's.
Moving on, though, because I have gotten you all riled up about A GIGANTIC RAINBOW SKITTLES CAKE THAT CAN FEED UP TO 20 PEOPLE, the cake has five vanilla sponge layers, each sandwiched by a neon, Skittles-flavored frosting.
On top of that, there are actual Skittles decorating the top of the cake, as well as a massive rainbow swirl. ASDA's supplemental information on the GIGANTIC RAINBOW SKITTLES CAKE THAT CAN FEED UP TO 20 PEOPLE is as follows: it costs 13 Euros (~$17) and is, well. It's a lot of calories. But it's a GIGANTIC RAINBOW SKITTLES CAKE THAT CAN FEED UP TO 20 PEOPLE. What did you think you'd signed up for?
More about the GIGANTI— you get it. More about the cake here.