Espheni and their purpose for invasion on Earth is being still a big mystery.
I had rewatch season 2 when Espehni were revealed to the audience. “Fisheads” – as they were called by the 2nd mass – were the first who made an offer for humans to transfer in safe area, which they called “New Jeruzalem”. It was not their intresting to extinct people, but they didn’t say what their intention was.When Tom was speaking with the Overload, the Espheni told Tom that if they wanted to make a genocide, all the people have been already dead . He said that this more like “Correction”. What this is mean? Does he mean that they were here before and now they’ve back to make correction? Maybe they are “Ancient Astronauts”? I can not rule it out, but for me more fits that it means they saw that people have been ruin their planet. Seems more like Espheni were looking for new home for them and they want to fix what people have destroyed in past centuries to make this planet more comfortable to stay. Perhaps they are running away from Volms, which are the real Aggressors.
“Fishheads” have found a good planet with lot of resources and much less developed species, so can easy handle with it. They watched the people and found that the resistance is much higher than assumed. Ben said to Tom (S2E7) that when Overlord was trying to reach his mind Ben felt that he is terrifying about Skitter’s rebellion. The Espheni didn’t expect such things like humans cooperating with rebel Skitters and it scared them. Maybe first they’ve only wanted to make their base on Earth, or even prepare the Earth as a kind of shelter from the Aggressors, but then they found out that people could be more useful than just as slaves. Making Karen as Overlord was also an exam, so they can see if she would be one of them, if her mind would handle with connection to the other Espehni and her physical body with harness. The humanity became a chance for “Fishheads” to survive in a new creations, a new kind of species which be more perfect and more able to win with Volms. But cooperation between humans and Skitters could ruin everything. Rebel Skitters made people to destroy the weapon against the Volms and let them to land on our planet. But maybe rebel Skitters haven’t known everything? Because first they help Volms and then Ben said to Tom, that the new leader of rebellions is not trusting the Volms. The question : why they destroy the weapon and let the Volms land? will stay without the answer.
When things started to slipped out of Karen’s hands she in act of desperation brought Alexia and Anne to Tom and wanted to tell him something important about the Volms. “There be more unpleasant discoveries about the Volm. This war has gone on for centuries. And it will continues for a long time to come. As you know from you own history, there might be a day when our interests will coincide” she said. But Tom shot her before she had chance to reveal more. He did what he need to do, but maybe also he lost the only opportunity to find out the truth. Well this is what we will never know.