In recent chapters, Vegapunk said that Joy Boy was the star of the story of the Void Century. He was born in the Great Kingdom 900 years ago at the beginning of the Void Century.
The Great Kingdom that predated the World Government was vast and possessed technological advancements that even Vegapunk genius could not attempt to replicate. This Kingdom probably functioned as a utopia: one where food and resources were abundant, diseases and poverty non-existent.
At some point, Joy Boy set out to the seas and became the first person ever to be called a pirate. Joy Boy was eventually defeated by the Twenty Kingdoms, which marked the end of the Great Kingdom and thus the Void Century.
Joyboy fighting the 20 Kingdoms was a big surprise, showing that the 20 kings had real combat skills, unlike many lazy kings in One Piece. Usually, groups don’t stand a chance against a single stronger enemy without some trickery (like Whitebeard vs. the Marines or Kaido vs. the Red Scabbards).
I think the 20 kings knew Joyboy was stronger and would win, so they performed a ritual to combine their essences into Imu. Imu represents the collective will of these kings, explaining why Imu uses “we” and wields one of the 20 swords. It’s similar to how Wapol needs the 20 doctors and merged with his subordinates for power.
I believe Imu wants Vivi to become complete because Lili refused the ritual, angering the other kings. Vivi resembles Lili, making her a suitable substitute.
Theory by Yxsh (https://www.reddit.com/user/Yash_15252)