Bowser's Lair is a custom track made by Thed0ra7z. It has the feel of GBA Bowser Castle 3, and features the koopa fireball, some really tight, jagged turns and a split route.
Ptpix1 released a remake of this track on January 17th, 2018. The remake features a similar layout, but with a number of tweaks so that it plays better. The remake also features animations, new textures, and is 3 laps instead of 2.