“… It’s a gaijin with a sword.”
The parasites have invaded the inhabitants of this world and absorbed their memories.
They know their host’s hobbies, relationships, bank account numbers, trade knowledge and skills, their fondest experiences, and their general knowledge of this world.
However, it was still an alien world to them.
They had to get used to having two arms and legs when they never had any before. Since they have the memories of hosts, they could walk, run and pick up things smoothly just like a regular person.
There was no problem with control. It just felt uncomfortable.
It was sort of like having a new prosthetic arm that was 100% accurate and precise, but it didn't feel like yours.
Equipped with memories of this world, they shouldn’t be too surprised when things come their way. However, when they heard that one of their own was killed by their prey who should be weaker than them, they were shocked.