This chapter was co-written by JeffC FTW
Inside of the Infinite Tsukuyomi, Mei Terumi dreamed of finally getting married while the other Kage attended the ceremony. The man she was married to...she didn't really remember that much. The only thing she could remember was how handsome he was. The guests congratulated her...except for Tsunade who could only watch in bitter jealousy. Oh, well. Her loss.
Everything the Fifth Mizukage dreamed of and relished was only in her mind, while she was still wrapped up in one of a million cocoons attached to the Shinju.
The God Tree's power destroyed the woman's clothing and everything else, leaving her radiant nudity behind. But just like the other most beautiful woman, Tsunade, it would be remade into a body befitting of a soldier.
As years passed, all of Mei's chakra was very slowly drained, including her Lava Release jutsu as the first stage of her transformation proceeded. Later on, her body would be drained next, slowly shriveling up.
Everything left there once was of Mei Terumi, being a mummified husk that lost its freshness over time: her red-brown hair thinned, becoming lighter, her skin pasty white and shriveled like uncared for clothing, the matter stretching over to outline the bones, and her breasts sagged. Her brain became smaller, too, but alert for the orders of a female she could not see but obey. Telling her that she was never meant to be a leader, even though women in power were just as capable as the men.
Many months passed. During that time, the skeletal thing regained what it lost if upgraded from female to semi-male. The long hair shortened to the cut of a soldier's, mass but not too bulky muscles replaced hourglass curves and full breasts, and the hair darkened from auburn to wildlife green, as well as the skin cracking and the teeth becoming round. The eyelids opened, revealing poisonous yellow rather than wood brown.
When the cocoon snapped and unwrapped what was once a human being. Mei Terumi the Fifth Mizukage was gone forever. The independent woman who wanted a man to love her and support her career and choices. She was now White Zetsu, like the others who were once her comrades and subordinates. They would all serve Kaguya and protect her from the invaders coming for her chakra. Such was the duty of a soldier.