If Sukuna was a female, I can imagine that after she takes over Yuji's body and fought Megumi early in the series, she would take the advantage of her overwhelming power and force Megumi to take part in her baby making ritual after realising his potential

Although Yuji's body is male, Femkuna's transient vagina and uterus should still be able to be impregnated by Megumi's potential sperm and upon returning control back to Yuji, Femkuna should be carrying a baby in her Malevolent Womb back in her inner domain

I imagine that time passes differently between the real world and her inner domain, so she's able to raise her child, who we can call Sukumi and has inherited the vast CE reserves of her mother and the precious Ten Shadows Technique of her non-consenting father

As Sukumi grew in power, it became powerful enough to master the Ten Shadows, and when the time came to tame Mahoraga, she was able to accomplish the impossible deed with ease

As time passed, Femkuna discovered new emotions while raising her child, emotions she did not know she had before, a genuine care for someone other than her own self

In time, Sukumi was able to use Mahoraga to give Femkuna a blueprint to adapt to Yuji's cage, unfortunately, since the child was born inside of Femkuna's inner domain, it lacks its own real body, meaning that the mother and child would be separated even if Femkuna took control of Yuji's body

Femkuna bides her time, waiting patiently
Skip past Goodwill, Shibuya and Perfect Preparation, we arrive at the Culling Games where it is revealed that Megumi's sister is actually Yorozu, a crazy male sorcerer obssessed with suckling on Femkuna's four bountiful bosoms

Femkuna takes this opportunity to take over Yuji's body using the binding vow in order to take advantage of a distraught Megumi

Before Megumi is able to summon his divine general, Femkuna converts her child's soul into cursed energy and pours all of that cursed energy into her uterus, which she promptly rips out and shoves down Megumi's throat

Due to their intense sex kaisen earlier in the series, neither Femkuna nor Yuji nor Megumi perceive this as harm to anybody

Yuji could only watch in despair from the inside as his best friend morphed into the cursed child of Femkuna

Yuji has lost his teacher, his classmates, and his reason for fighting

As he struggled to usurp control from Femkuna, the Queen of Curses appears before him in her inner domain, revealing that she was now the cage and Yuji is but a trapped soul, cursed to spend an eternity inside Femkuna's inner domain

He would live forever, seeing everyone around him die