This design is easy, in fact is the same model i used for fecto forgo, i only changed the colors and added the tentacles it has.

After Forgo Leon is defeated, Fecto Forgo jumps out of his body and transforms into Soul Forgo. Then Soul Forgo is about to attack Kirby and Elfilin, but is stopped when a butterfly (Morpho knight) appears, and absorbs Soul Forgo's power.

As there is not much information about soul forgo (cuz it only appears in one cutscene) im leaving fecto forgo's wiki instead:https://kirby.fandom.com/wiki/Fecto_Forgo

:)