In Hollow Bastion in the fight against the Heartless, Rikku turned on Yuffie. B*tch took her spot and she couldn't handle it. Aerith keeps the peace. She could not allow this at this time. So she used her magical spells but Rikku totally kicked her off the mountain and thankfully Cloud caught her. But Sephy stole her and stabbed her virgin body while the blood spilled on Cloud. Cloud then gained a magical barrier from her love. And Tifa could not take this sh*t. So she dropped kicked Sephy for making Aerith's love touch Cloud in a way her love could never touch him. Then Aerith's body split in half. Sephy, Aerith's head, and Tifa unfortunately fell into a crevice filled with Darkness and Heartless.
Cloud mourns this day with his new black clothes and keeps Aerith's bottom half very, very, very close to him...while he cries...while he cries T_T
He swears the day he finds Sephy again it will be his last...because he knows Sephy survived because he can fly.