From the Super Mario Wiki, the Mario encyclopedia
The Bowser Monument is a giant, flaming, living Bowser Statue in Mario Kart 8 and Mario Kart 8 Deluxe. It is the centerpiece of Bowser's Castle, and it is featured in the course's intro. As the racers drive past it, the Bowser Monument punches into the track, sending ripples through the track that cause it to continuously ripple. The track splits into two, with the Bowser Monument in the middle of them, and it alternatively punches each side of the track.
It greatly resembles the other large Bowser Statues from Mario Kart 64 as well as Mecha Bowser from Mario Kart: Double Dash!! and Mario Kart Wii, but intermixed with lava, and punching instead of shooting fire.