Cursa, having descended from the Megabug encountered in your previous adventure... ...has inherited its memories, from which it reconstructed this likeness of Bowser.JEANIE on Darkmess Bowser
Darkmess Bowser is a minor antagonist in Mario + Rabbids: Sparks of Hope. It is one of the second to last foes encountered at Cursa's stronghold before the battle with Cursa herself. It takes on the very likeness of Bowser himself.
Biography
The MegaBug had once taken full control of Bowser and its memory of everything has been passed down to a small fragment that floated aimlessly through the galaxy and merges with space debris, gaining sentience over time and eventually becoming Cursa. After it takes control of Rosalina, it creates the Spark Hunters along with its own minions and corrupts Bowser's army as well leading the Koopa King to get involved as well.
When the heroes arrive at Cursa's stronghold, they encounter two Darkmess Puddles they have to cross before they reach the golden cannon to reach Cursa itself. Darkmess Bowser appears in the first Darkmess Puddle where Beep-0 recalls the time they fight him when he was possessed by the MegaBug and JEANIE backs up the story that Cursa inherited its memories from its predecessor of the events of Kingdom Battle and recognized Bowser enough to create it. The real Bowser was itching for a fight against his clone and is required for this battle. After it is destroyed, the next battle is against Darkmess Edge.