Essence extraction
Cum is the essence of life
Chapter 1
by 
Popa2112
Explanation:
On their 18th birthday some people get a set of powers that allow them to transform their partners. There is no way to know if someone has the ability other than the results.
If the person with the ability cums while thinking of a physical or mental trait then that cum can cause said transformation with no detriment to the powered person. if the powered person's partner cums then whatever trait the powered person is thinking of will be extracted from the partner into the cum.
(hopefully this is a better explanation)
Also feel free to add your own content anywhere i would love to see where you guys would take this concept.