Biollante is the result of the unnatural splicing ... were woken by the Oxygen Destroyer, the weapon used to kill Godzilla in the original 1954 movie, which also caused them to mutate and grow ...

When a scientist’s daughter is killed, her consciousness becomes intertwined with a plant-based monster dubbed Biollante. When fighting Godzilla, Biollante takes on a more crocodile-like form ...