"The soul is the single greatest threat to our species. If it's there I'll find it, and I'll destroy it"
The Human Soul is a spiritual being underneath and present in humans.
The Human Soul?Edit
The human soul is what is underneath every human and is the well spring of human emotion. It what makes a human a human. And after death, it is believed the soul will live on, meaning it's immortal and everlasting. It also can't be explained with just science and can't be controlled.
When Diana was High Commander of the Visitors, she sent a sleeper agent to the Vatican to learn more about human emotion and it's affect on V species. There he and later Diana learned about the soul. Both believing the soul to be a blessing and a gift and could sustain the Visitors forever. When Anna took control, she dismissed the soul as fantasy and nothing more, only till seeing her mother again that she now knows the soul is real. Rather seeing it as a gift, she believes it to be the greatest threat.
At first, Anna did not understand the soul, because of the fact she has a scientific mind, and believes that it could be held in a test tube, but it's more complex than that.
There she uses the humans who have been abducted to be used as experiments to locate and extract the soul to find ways on destroying it. The resulting cost would be hundreds of thousands of humans for the experiments to be fulfilled, because each experiment ends with death.
At the end of Concordia episode and onward, the invasion plan is back on track, but all that is left is to conquer the soul, because that if not removed then the the next generation of V's would carry the soul, because they'll have human DNA.