Note: this post contains info not revealed until episode 42. Spoilers.
[HIDE]Whenever the forest invades a world a golden/forbidden fruit is created. This fruit is given to a 'Woman of the Beginning' who chooses a champion to claim it and bring about a new era (forcing that species to evolve further in the process, which is what the forest wants).
The OverLord Queen was their world's 'Woman of the Beginning' and chose to give their fruit to her husband, Roshuo.
When the forest started invading Earth, Roshuo tracked down Earth's fruit before any human could, to keep them from getting it.
Roshuo extracted part of the power of the Earth's fruit and gave it to Sagara, who turned it into the Kiwami lockseed. The kiwami lockseed doesn't have the full power of the fruit but it seems to be growing in strength, slowly turning Kouta into an OverLord in the process.
Roshuo kept the rest of Earth's fruit until episode 41, where he placed it into Mai's body. She is now becoming the 'Woman of the Beginning' for Earth.
When Roshuo was defeated by Redyue, Redyue pulled a fruit out of his body. However it was black and rotten. Redyue seemed to think it was a decoy but it's not been said for certain if that's true, if Roshuo had simply used up all of the power in his world's fruit, or if it was because Redyue wasn't the one chosen by the OverLord Queen.[/HIDE]