The phrase "frog king" refers to a king who is turned into a frog, typically through a magical spell or curse. It might also signify a frog with king-like qualities or traits. Full definition
Instead of fighting Bowser and his army of Koopas, this time Mario, Luigi, Peach and Toad are placed in a similar, yet different, setting called Subcon (also known as «the land of dreams»), that was once a lovely place for living and a real wonderland, just like the Mushroom Kingdom, but it is now ruled by the giant frog king called Wart, and peace is no longer residing in this area. (superluigibros.com)
There are a couple of nice comic interludes, most notably John Cleese's dying Frog King routine, and the kids will be duly diverted, but it all smacks of a general downward spiral; there are no music choices as bold as the original's iconic «Hallelujah», and no puns as audacious as calling the villain «F**k wad» (OK, technically «Farquaad») and still getting a family - friendly rating. (theguardian.com)
The evil Frog King and his army are trying to steal the magic mushrooms! (toucharcade.com)